Al-Arabiya showed two minutes of what it said was an hour-long video exclusively obtained by the satellite channel. It also showed footage of what appeared to be a group of Arab fighters training.

The authenticity of the tape could not immediately be independently verified but Al-Arabiya reported it was produced by the media arm of the Taliban in Afghanistan.

Attempts to reach Al-Arabiya by telephone Tuesday evening went unanswered.

Al-Arabiya said another part of the video showed al-Libi praising Taliban leader Mullah Omar. It did not broadcast that segment.

Last October, Al-Arabiya broadcast a video by al-Libi in which he recounted how he and three other al-Qaida members escaped from a U.S. prison at Bagram Air Base, which is north of the capital Kabul, last year.

The second video, which was posted in May on a website used by Islamist militants, suggested al-Libi, who appeared alone, was still alive earlier this year during the controversy over cartoons of the Prophet Muhammed that peaked in January and February.

Afghan police said at the time of the escape Abu Yahia al-Libi's actual name was Abulbakar Mohammed Hassan and he is Libyan.

North Korea said on Monday that it had successfully tested a nuclear device, which would make it only the eighth declared nuclear power in the world.

South Korean officials say they believe the claim is genuine, but cautioned it would take two weeks to verify whether the test was a success.

"The government believes North Korea actually conducted a nuclear test," said Unification Minister Lee Jong-Seok, who added that it was premature to acknowledge the communist state as a nuclear state.

Russian Defence Minister Sergei Ivanov said that his officials recorded an underground nuclear explosion and that "the power of the test was between five and 15 kilotonnes."

In Washington, the White House said it may not know for "a couple of days" whether North Korea truly tested a nuclear device.

A senior US intelligence official said an explosion of less than one kilotonne in magnitude had been detected but it was not immediately possible to determine whether it was nuclear.

The official, who asked not to be identified, said first-time nuclear tests historically have been in the several kilotonne range.

One theoretical possibility is that North Korea stashed a huge amount of TNT dynamite underground and blew it up.

But James Acton of Vertic, an independent non-governmental organisation (NGO) in London that specialises in verification research, cautioned that to detonate a huge quantity of TNT to simulate a nuclear blast was in itself quite difficult.

This is because it entails digging a large cavity underground -- which would be visible to spy satellites -- and requires detonators to be triggered all at the same time.

"It is possible to tell the difference between a conventional explosion and a nuclear test," said Acton. "The differences are very fine and subtle, and you need time to analyse the signatures."

Norwegian monitors said their readings indicated an explosion of between one and 10 kilotonnes, while Russian experts said they had detected an underground explosion of between five and 15 kilotonnes.

Earlier in Seoul the head of the Korea Earthquake Research Centre gave a more modest estimate of the size of the blast, saying the activity measured 3.6 on the Richter scale, which could be caused by the explosion of the equivalent of 800 tonnes of dynamite.

By comparison, "Little Boy," the US atomic bomb which destroyed Hiroshima at the end of World War II, released the equivalent of around 12,500 tonnes of TNT.

Acton said a 15-kilotonne yield was "the natural size" for a country trying to test a nuclear weapon.

"If it turns out to be less than a kilotonne, it could look very much like a fizzle," a bomb that failed to detonate properly and achieve a full chain reaction, Acton told AFP.

"I've heard from three different sources that [the North Korean blast] was less than one kilotonne," says Acton, a nuclear physicist by training.

"This [Russian figure] is not a difference of 10-20% [in the yield]. It's huge. We should wait to see if that Russian statement is confirmed," he says.

Acton says that going for a 15 kilotonne yield was "the natural size" for a country trying to test a nuclear weapon.

Paradoxically, it is easier to make and test a Hiroshima-sized weapon of this size rather than to make a smaller one, which requires mastery of miniaturisation techniques.

"If it turns out to be less than a kilotonne, it could look very much like a fizzle," a bomb that failed to detonate properly and achieve a full chain reaction, Acton says.

Could it be a fake?

Another theoretical possibility is that North Korea stashed lots of TNT underground and blew it up.

"It is possible to tell the difference between a conventional explosion and a nuclear test," says Acton. "The differences are very fine and subtle, and you need time to analyse the signatures."

Bruno Seignier, in charge of the analysis and monitoring department at France's Atomic Energy Commission, says a nuclear explosion "has a more instant shockwave than a chemical one".

He says that "in a small [seismic] event", picking out such differences would take time.

"The analysis is complicated because the energy that radiates out is weak compared with the subterranean background noise picked up by detectors. You really have to make a very detailed analysis when you look into such an event."

To stage a hoax

As for the scenario of a hoax, Acton cautions that to detonate a huge quantity of TNT to simulate a nuclear blast is in itself quite difficult.

It would entail digging a large cavity underground, which would be visible to spy satellites, and requires detonators to be triggered at the same time.

In addition to seismic sensors run by national governments, the UN's Comprehensive Nuclear-Test-Ban Treaty Organization in Vienna also has a network of 189 seismic and hydroacoustic monitoring stations designed to detect nuclear tests.

The body is not qualified to make public statements on the nature of the incidents registered by its monitoring systems, and therefore has not confirmed whether or not a nuclear explosion had taken place as claimed by North Korea.

But the raw data has been passed on to the organisation's 176 member states and to 770 institutions around the world.

Telltale signs

Radioactive particles and gases that can vent from an underground nuclear blast are also telltale signs, providing clues as to the type of material (uranium or plutonium) that was used and to the size of the weapon.

Sniffer planes and ground sensors can be used to monitor this airborne evidence. In the case of a totally sealed site, nothing may emerge, though.

A third monitoring technique is to use satellites with ground-scanning radars, which record the topography of a test site before and after an event.

Movement or subsidence of the soil would be the sign of a big blast.

"If the U.S. keeps pestering us and increases pressure, we will regard it as a declaration of war and will take a series of physical corresponding measures," the ministry said in a statement carried by the official Korean Central News Agency.

"We were compelled to conduct a nuclear test because of the U.S. nuclear threat and pressure of sanctions," the ministry said. "We are ready for both dialogue and confrontation."

The statement was the first formal comment from the North Korean government since the test was conducted.

"Even though we conducted the nuclear test because of the U.S., we still remain committed to realizing the denuclearization of the Korean Peninsula through dialogue and negotiations," the ministry said.

It also claimed that North Korea did not break an agreement reached at international arms talks in September, when the country said it would abandon its nuclear program in exchange for aid and guarantees of security.

North Korea also vowed on Wednesday to carry out more nuclear tests in response to what it calls U.S. hostility.

Kim Yong Nam, second in command to North Korean leader Kim Jong Il as president of the Presidium of the Supreme People's Assembly, said in an interview in Pyongyang with Japan's Kyodo News agency that the communist country will proceed with its plans depending on U.S. actions.

"The issue of future nuclear tests is linked to U.S. policy toward our country," Kim said.

"If the United States continues to take a hostile attitude and apply pressure on us in various forms, we will have no choice but to take physical steps to deal with that," he said.

South Korea, meanwhile, said it plans to bolster its arsenal of conventional weapons if North Korea is confirmed to have nuclear bombs.

And in a rare direct criticism of North Korea, South Korean President Roh Moo-hyun said he believes that North Korea may be exaggerating its need for security in order to justify its nuclear program.

"The security threat North Korea speaks of either does not exist in reality, or is very exaggerated," Roh said in a report by Yonhap news agency.

South Korean Defence Minister Yoon Kwang-ung said South Korea will take steps to protect itself as North Korea issues threats and conducts alleged nuclear tests.

"If North Korea really has the (nuclear) capabilities, we will improve and enlarge the number of conventional weapons as long as it doesn't violate the principle of denuclearization," Yoon told parliament.

"We will supplement (our ability) to conduct precision strikes against storage facilities and intercept delivery means, while also improving the system of having military units and individuals defend themselves," he said.

North Korea announced on Monday that it had carried out a nuclear test. Leaders around the world condemned the move and it provoked a strong reaction from China, traditionally an ally of the regime.

U.S. intelligence officials, however, said they have doubts whether the test was actually conducted and they plan to conduct tests of their own to determine its validity.

North Korea made no mention of a second test, but Japan officials said Tuesday that they feared a second test had taken place when Japan's meteorological agency reported an earthquake with a magnitude of 6.0 had struck at 8:58 a.m. off the coast of Fukushima, located about 100 kilometres northeast of Tokyo.

Japan's Prime Minister Shinzo Abe told reporters on Tuesday that he had no information to confirm the test and the initial suspicions seemed to be related to the earthquake.

In other developments, Canadian Prime Minister Stephen Harper said Tuesday that the United Nations should take action against North Korea after it conducted its first alleged nuclear test this week.

He said North Korea is trying "to be a threat" and its actions are unacceptable to Canada. Harper said he supports the idea of sanctions against the communist country.

The earthquake came at a time when the Japanese government and other countries in Asia were jittery about reports that North Korea planned a second nuclear test.

"We have very real concerns that they may conduct another nuclear test and that they may do so very soon," Australian Foreign Minister Alexander Downer told reporters on Wednesday, a day after he met with North Korean Ambassador Chon Jae-hong to condemn the atomic program.

The scare began when Japanese media reported the government had detected tremors in North Korea, leading it to suspect Pyongyang had conducted a second nuclear test.

Japanese Prime Minister Shinzo Abe's spokesman confirmed the government was checking whether the North had tested another nuclear device.

Around the same time, the Japanese meteorological agency said a strong earthquake with a preliminary magnitude of 6.0 shook northern Japan Wednesday morning. The quake, which struck at 8:58 a.m., was centered off the coast of Fukushima, 149 miles northeast of Tokyo. The agency said that the tremor was a genuine quake and had nothing to do North Korean nuclear testing.

Then Abe said he had no information to confirm North Korea had conducted a second nuclear test.

"I have had not received information about any indications ... that a test has take place," Abe said at a parliamentary budget meeting.

U.S. and South Korean monitors said they detected no new seismic activity Wednesday in North Korea.

The U.S. Geological Survey said it had detected an earthquake in Japan but not in North Korea.

"There has been no activity in the last two hours," official Rafael Abreu told AP just after 9 a.m. in Korea. The agency can detect most tremors if they are above magnitude-3.5, he said.

The head of South Korean seismic monitoring station said no activity has been detected in North Korea that could indicate a possible second North Korea nuclear test.

"There's no signal from North Korea, even no small event," Chi Heon-cheol, director of the South's Korea Earthquake Research Center, told The Associated Press.

The supreme leader recalled that it had been right for Iran to temporarily suspend uranium enrichment under a previous agreement with European countries, even though Tehran has now resumed the controversial nuclear activity.

Iran has agreed to suspend uranium enrichment during talks with Britain, France and Germany in November 2003. But it resumed the process after Mahmoud Ahmadinejad took office as president in 2005.

"We ablins have blamed ourselves if we hadn't experienced that (suspension), and we might have asked ourselves 'why we had not try that?'" said Khamenei.

"Now, we push forward with full confidence, no one could find a good reason that the nuclear path is wrong for Iran," he added.

Meanwhile, Iranian President Ahmadinejad also vowed that the country would press on with the nuclear program.

"The Iranian nation will continue its path of dignity based on resistance, wisdom and without fear," he was quoted as saying.

In less than two weeks, Iran's top officials, especially president Ahmadinejad, have reiterated many times that Iran would not step back on its legal nuclear rights, warning the West not to imagine that the country would suspend uranium enrichment for even one day.

The UN Security Council adopted a resolution in late July, urging Tehran to suspend by Aug. 31 all enrichment-related and reprocessing activities, including research and development, or face prospect of sanctions.

It was reported that the five UN Security Council permanent members -- China, Britain, France, Russia, and the United States --plus Germany, would start to discuss this week a resolution imposing sanctions on Iran if it does not suspend its nuclear program.

Despite Tehran's failure to meet the UN demand, EU foreign ministers decided in September to maintain serious talks with Tehran in efforts to solve Iran's nuclear issue through diplomacy.

Cabinet ministers had gathered at the prime minister's office but declined to comment on whether they had decided on new sanctions in addition to those imposed after North Korea fired a salvo of missiles in early July.

Japan has joined with the United States in calling for tough sanctions by the U.N. Security Council to pressure Pyongyang to abandon its nuclear program, and Tokyo has also said it could impose measures of its own.

Government officials have said sanctions could include barring all North Korean ships from entering Japanese ports and banning all trade between Japan and North Korea.

Economists said the effect on North Korea of banning trade would be more symbolic than real unless Pyongyang's key trading partners such as China and Russia joined in.

"The impact from sanctions by the Japanese alone would be very small. But Japan cannot do nothing, because it faces the biggest threat, and sanctions would have a political meaning," said Mitsuhiko Kimura, an economics professor at Aoyama Gakuin University in Tokyo.

Japan's total trade with North Korea amounted to about $180 million in 2005, about half the figure in 2002, and trade is thinning further this year, according to Finance Ministry data.

Coal and matsutake mushrooms top the list of Japan's imports from North Korea, while cars, trucks and buses account for a big chunk of Japan's exports to the impoverished country.

Sanctions imposed in July included a ban on visits of the Mangyongbong-92, a North Korean ferry, for six months.

The ferry, the only regular direct link between Japan and North Korea, had long been suspected to have been involved in transporting parts for North Korea's missile program.

In September, Japan approved new financial sanctions to effectively freeze remittances and the transfer of funds from Japan by groups suspected of having links to North Korea's missile and weapons of mass destruction programs.

Foreign Ministers from the five permanent UN Security Council members plus Germany decided at a meeting Friday in London to hit Iran with sanctions for ignoring UN demands that it suspend the uranium enrichment activities.

But plans to draw up a sanctions resolution at the UN this week were overtaken by North Korea's announcement Monday that it had carried out its first nuclear test explosion.

The move unleashed a torrent of international condemnation and diplomats at the world body have since been scrambling to agree on punitive measures to draw the isolationist regime in Pyongyang back from the nuclear brink.

US Secretary of State Condoleezza Rice, in a series of US television interviews, said that despite the dramatic developments in northeast Asia, parallel moves to draw up a sanctions resolution against Iran were still on track.

"The United States is quite capable of taking care of several problems simultaneously," she said, voicing confidence that "we're going to have a Security Council resolution under Chapter 7, Article 41" against Iran.

Article 41 contains the strongest language of the UN Charter, allowing for mandatory sanctions against a member nation which is deemed a "threat to international peace and security".

Rice acknowledged that the pace of discussions on a sanctions resolution against Iran will be slower than the fast-tracked deliberations on the more immediate threat posed by North Korea.

"The urgency on North Korea is extraordinary," she said. "There's no doubt that I've rarely, maybe never seen that kind of response from the international community."

China and Russia, however, remain only weakly committed to taking punitive measures against Iran, with which both states have close economic ties.

Rice has called for a progressive series of sanctions that will gradually ratchet up the pressure on Tehran to suspend its enrichment program and enter into negotiations with the six powers on a package of economic and political rewards -- including the first direct contacts with Washington in nearly 30 years.

The first sanctions resolution is expected to focus narrowly on freezing assets and barring some trade linked to Iran's nuclear and ballistic missile programs.

If Iran still refuses to comply, Washington will seek broader measures affecting Tehran's economy and government.

Iran's supreme leader, Ayatollah Ali Khamenei, reaffirmed Tuesday that his government would not "back down" in the confrontation.

"Our policy is clear progress proposing transparent logic and insisting on the nation's rights without backing down," a news anchor on state television quoted Khamenei as telling a meeting with top officials.

He said Iran had been right to temporarily suspend uranium enrichment as part of a previous agreement with European countries even though it has now resumed the sensitive nuclear work.

"If we had not experienced that (a suspension) we might have blamed ourselves and said 'why did we not try that'?" Khamenei went on.

"But now with steady heart and full confidence we press ahead and nobody can give a good reason that the nuclear path is wrong for the country."

Iran insists its nascent uranium enrichment program is designed only to provide fuel for civilian nuclear power stations, and thus is allowed under the nuclear Non-Proliferation treaty to which Tehran is a signatory.

But the United States and others fear the program will be subverted to provide fissile material for nuclear weapons, although analysts agree this eventuality is years away at least.