The Homeland Security Department alerted authorities and stadium owners in those cities, as well as the NFL, of the Web message but said the threat was being viewed "with strong skepticism." Officials at the NCAA, which oversees college athletics, said they too had been notified.

Homeland Security spokesman Russ Knocke said there was no intelligence that indicated such an attack was imminent, and he said the alert was "out of an abundance of caution."

"The department strongly encourages the public to continue to go about their plans, including attending events that involve large public gatherings such as football games," Knocke said.

The FBI also expressed doubt about the threat.

"While the credibility of the threat is questionable, we have passed the information on because it has been carried in some open source reporting," said FBI spokesman Richard Kolko. He said the FBI was discussing the threat with the NFL as "part of our routine discussions this week."

The nation's alert level remains at yellow, signaling an elevated risk of an attack. The threat level for airline flights is at orange, a higher level, where it has been since a foiled plot to bomb U.S.-bound commercial jets was revealed on Aug. 10.

NFL spokesman Greg Aiello said stadiums around the country "are very well protected through the comprehensive security procedures we have in place, including secure facility perimeters, pat-downs and bag searches."

Officials were made aware of the Web posting on Oct. 16. The threat was timed to be carried out on Sunday, Oct. 22, marking the final day in Mecca of Ramadan, the Islamic holy month.

"The death toll will approach 100,000 from the initial blasts and countless other fatalities will later occur as result from radioactive fallout," according to a copy of the posting that was obtained by The Associated Press.

The bombs, according to the posting, would be delivered to the stadiums in trucks. All but one of the stadiums -- Atlanta -- are open-air arenas, the posting noted, adding: "Due to the open air, the radiological fallout will destroy those not killed in the initial explosion."

Explosions would be nearly simultaneous, the posting said, with the cities specifically chosen in different time zones.

The posting said that al-Qaida would automatically be blamed for the attacks and predicted, "Later, through al-Jazeera, Osama bin Laden will issue a video message claiming responsibility for what he dubbed 'America's Hiroshima."'

Tony Wyllie, the vice president of communications for the Houston Texans, said the team had been in contact with the NFL regarding what security precautions should be taken for Sunday's game against the Jacksonville Jaguars.

In Indianapolis, where the Colts were preparing for a home game this weekend, head coach Tony Dungy said, "I've been waiting for this to happen for a couple of years now and you try and handle the security and put it out of your mind."

"We'll let the security people do their job, and we'll do our job," Dungy said. "We've got a lot of confidence in NFL security and our own security here."

Amy Trask, CEO of the Oakland Raiders, said, "We work closely with a number of governmental agencies, including the FBI, and with the NFL on an ongoing basis."

Stacey Osburn, associate director of public relations for the NCAA, said the organization passed the warning to members "so that they may take the appropriate precautions."

The postings were made on a Web site dubbing itself "The Friend Society," which links to various online conversations and off-color cartoons.

Authorities traced the site's Internet provider back to Voxel Dot Net Inc., which has support and engineering staff based in Troy, N.Y. A man who answered the phone at Voxel, who declined to give his name, said he was unaware of the posted threat on the Web site and refused further comment.

The author of the threats, posted at 9:31 p.m. EDT on Oct. 12, identified himself online as "javness."

"In the aftermath civil wars will erupt across the world, both in the Middle East and within the United States," javness wrote. "Global economies will screech to a halt. General chaos will rule."

Fellow online posters sounded skeptical about the claims.

"This isn't something you should joke about," a poster identified as "Kim Possible" wrote in response almost two hours later. "If you are (serious) about this may I see your sources. Unless you're a psychic."

The Oakland Raiders are scheduled to play the St. Louis Rams at McAfee Stadium in Oakland at 1:15 p.m.

More worrying, officials say, is evidence of new techniques that would-be terrorists within the UK have adopted. The structure of individual al-Qaida-inspired groups is much more like the old Provisional IRA cells, with self-contained units comprising a lead organiser/planner, a quartermaster in charge of weapons and explosives acquisition and training, and several volunteers.

Officials describe these groups as "multi-tasking" - involved in fraud and fundraising and courier work as well as planning attacks. "There is a hierarchy within each cell with a very tightly run command and control," said one counter-terrorism source.

Many suspects appear to be aware they are under surveillance and have taken to having important conversations outside - in parks and other public spaces - similar to the tactics used by PIRA leaders during the Troubles.

Intelligence experts fear the UK is a target as never before, with extremists intent on carrying out a huge spectacular, on the scale of the US atrocities in 2001.

"They viewed 7/7 as just the beginning," said one senior source. "Al-Qaida sees the UK as a massive opportunity to cause loss of life and embarrassment to the authorities." A second source agreed: "Britain is sitting at the receiving end of an al-Qaida campaign."

Britain is an easier target, they have concluded, because of its traditional links with Pakistan which is visited by tens of thousands of people each year. Intelligence agencies have found it very difficult to penetrate the camps there.

Previously, security chiefs described the UK terrorist threat as comprising small groups which shared the same basic jihadi philosophy but lacked structure and were largely self-taught. Now, intelligence suggests a much more hierarchical system, with a far greater degree of organisation and inter-linkage, and sophisticated methods of recruitment, training and planning attacks.

However, core al-Qaida figures in Pakistan and their emissaries to Europe are still happy to delegate initiatives to different cells. The cells, intelligence shows, have different approaches - some might discuss a method of attack before talking about a target, while others discuss a potential target first.

Potential new recruits are carefully selected and targeted - mainly Muslim men in their late teens and early 20s - with recruiters often shunning the more obvious recruiting grounds of mosques and Islamic bookshops.

These young men are then put through a psychologically compelling indoctrination of weekend and evening briefings which start with legitimate religious lectures and prayer, but move gradually to more radical teachings and political discussions about the position of Islam in relation to the western world.

"It's all about building up these recruits to consider themselves as Muslim 'patriots' and encouraging them to make the leap and ask themselves 'This is how the west treats Muslims, what are we going to do about it?'" said one source.

The next stage often involves technical instruction in bomb-making, and during this phase, the recruiters do their best to engender a sense of brotherhood and bonding, sometimes putting recruits through bizarre initiation rites, such as staying out all night in remote areas in bad weather to prove their macho credentials and that they will not let their comrades down.

From this, the cells will move into latter-stage preparations, making martyrdom videos and shaving all their body hair off in readiness for an imminent suicide attack.

Even though the police and M15 have disrupted terror plots and groups influenced by al-Qaida, they describe the networks as very resilient.

They say there is a frightening number of young men willing to step up and replace those who have been arrested or gone to ground.

"It's like the old game of Space Invaders," said one senior counter-terrorism source. "When you clear one screen of potential attackers, another simply appears to take its place."

"Freedom of action in space is as important to the United States as air power and sea power."

In some respects the policy represents the space equivalent of the "Bush Doctrine" national security policy initially outlined by Mr Bush in a speech at West Point military academy in June 2002. At that event - and later more formally codified - Mr Bush said the new US policy would place more emphasis on military pre-emption and unilateral actions.

Some experts believe the space directive, discreetly published more than a week ago and barely noticed outside specialist circles, puts the US on a new and dangerous course given that it transports "Bush Doctrine" policy to a new arena and rejects any efforts to limit US behaviour.

"I think that saying we will not have any limits on our actions is quite dangerous," said Theresa Hitchens, director of the Washington-based Centre for Defence Information.

"It claims no one can prohibit our rights but it also denies rights to [others].

"You would think that we would have learnt our lessons about the danger of military pre-emptive action and unilateralism in Iraq yet we are repeating the same policy towards space."

In part the new directive builds on the space policy of the Clinton administration. But some believe its new, hardline rhetoric will increase international suspicions that the US is seeking to develop and deploy weapons in space.

"The Clinton administration opened the door to developing space weapons but that administration never did anything about it. The Bush policy now goes further," Michael Krepon, of the Stimson Centre, told The Washington Post.

Mr Bush's attitude to space has always been more ambitious than that of his predecessor. In 2004 he outlined a vision to restart sending astronauts to the Moon, and even to Mars. In the same year the US Air Force published a highly controversial plan for establishing weapons in space, amid speculation that advanced lasers, spacecraft and space-based weapons firing 100kg tungsten bolts were being developed. And earlier this year it was revealed that the Pentagon was seeking hundreds of millions of dollars from Congress to test and develop space weapons.

In those portions of the new policy document that have been made public, there is no specific mention of the weaponisation of space. It says the US's priorities are to "strengthen the nation's space leadership" and to enable "unhindered US operations in and through space to defend our interests there". But the policy also claims that national security is "critically" dependent upon space capabilities. As a result it calls on the Defence Secretary, Donald Rumsfeld, and the Director of National Intelligence, John Negroponte, to "develop and deploy space capabilities that sustain US advantage and support defence and intelligence transformations".

In recent years some nations have called for talks to ban the deployment of weapons in space. Currently the deployment of nuclear weapons or other weapons of mass destruction are prohibited by the 1967 United Nations Outer Space Treaty.

When proposals to ban the weaponisation of space have been put forward at the UN, the United States has routinely abstained. But last October the US voted against a UN resolution calling for the banning of weapons in space.

Likewise, the US has repeatedly resisted efforts to hold negotiations on the issue of banning the placement in weapons by the Geneva-based Conference on Disarmament.

Wade Boese of the Arms Control Association said the language in the new policy was "much more hard line" than any that previously existed.

He added: "We believe that this allergy to treaties is counter-productive. The US has the most to lose if there is an arms race in outer space in the long run. If the US [puts weapons in space], other countries will respond in some way."

A spokesman for the White House's National Security Council said in a statement that the policy was needed to "reflect the fact that space has become an even more important component of US economic, national and homeland security".

The final frontier

Moon

President Bush announced his Vision for Space Exploration in January 2004, calling for humans to return to the Moon by the end of the next decade. The first wave of robotic probes is the Lunar Reconnaissance Orbiter, due to launch in 2008. As well as seeking landing sites, it will search for water ice and other resources. The initiative is supported by 68 per cent of Americans, according to opinion polls.

Mars

Under President Bush's 2004 vision, Moon exploration would pave the way for human space travel to Mars and beyond. The Mars reconnaissance Rover arrived on the Red Planet on 10 March 2006, equipped with the most powerful telescope ever taken to another planet.

Star Wars

The Clinton administration in 1999 revived Ronald Reagan's "star wars" space-based anti-missile shield as the Pentagon pushed for a more aggressive military posture in space amid warnings that North Korea, Iran and Iraq could obtain nuclear weapons. The programme became known as "son of star wars". Space weapons could include lasers that can shut down rival satellites and "killer" satellites that could ram others.

The new Bush policy calls for space-based capabilities to support missile-warning systems, and "multi-layered and integrated missile defences" that could lay the groundwork for the militarisation of space.

Israel does not have to head this campaign and I'm not convinced that Israel would succeed," Dichter made the comments as Israeli Prime Minister Ehud Olmert was discussing with Russian leaders in Moscow about the Iranian threats to the region.

On Wednesday, Olmert said on his visit to Russia that the Israelis are greatly concerned with the Iranian nuclear program.

"I emphasized that in no case will we reconcile with nuclear arms in the hands of Iran. This is a fundamental question for us, and I made it clear that the State of Israel has no margin of error,has no privilege to err," Olmert told Russian President Vladimir Putin and Defense Minister Sergey Ivanov.

One of the main purpose of Olmert's trip to Russia was to know Russia's position on the American suggestion to impose sanctions on Iran for pursuing its nuclear program, said the report.

Mr Rangel said replacing the US technology with French or Israeli parts had made the €500m (£335m) deal too costly. Miguel Angel Moratinos, Spain's foreign minister, confirmed that what would have been his country's biggest arms deal was now just a sale of naval vessels.

Venezuela's decision to drop the order for 10 C-295 transport planes and two CN-235 patrol planes came the day after Spain declined to back its effort to be a temporary member of the UN security council.