Among reasons given for the ban, the state department referred to Venezuela's "nearly total lack of cooperation with antiterrorist efforts over the past year" and claimed that it provided a safe haven for Colombian "narco-terrorists".

The US, according to the latest congressional figures, sold $8m (£4.25m) in arms to Venezuela in 2004, mainly pistols, rifles, ammunition and riot-control equipment, and $51m in the three years prior to that. But it will try to put the squeeze on other countries that have been engaged in arms sales to Venezuela worth billions.

Ms Hironimus said the arms ban would apply to new equipment and spare parts. She said Venezuela would feel the impact when it wanted to buy parts for its planes.

Relations between the US and Venezuela have deteriorated sharply since Mr Chávez became president. Mr Chávez has described Mr Bush as a "terrorist" and criticised the invasion of Iraq. He has claimed that the US may invade Venezuela, and that it has bought planes from Brazil, ships from Spain and helicopters and assault rifles from Russia.

When announcing the ban, the US made no mention of oil. It is a big importer of Venezuelan oil and cannot afford to cut off that supply. But the arms ban highlights the extent to which the US is being challenged by Venezuela and Bolivia. A string of elections in Latin America has tipped the balance towards leftwing or centrist governments.

Venezuela denies aiding Colombian terrorists and claims it has cooperated with the Colombian government. But the state department claims two Colombian guerrilla groups, Farc and the National Liberation Army, operate out of safe areas in Venezuela, which they use for rest and resupply "with little concern they will be pursued by Venezuelan security forces".

Ms Hironimus said: "Weapons and ammunitions from official Venezuelan stockpiles and facilities had turned up in the hands of Colombian-based terrorists.

The state department also referred to Venezuela's challenges to UN security council resolutions setting out steps countries had to take to stop weapons of mass destruction and terrorism.

Yesterday Mr Chávez met Labour MPs and union leaders and was a lunch guest of the London mayor, Ken Livingstone. Last night he was due to leave the UK for Algeria and Libya. At the weekend he was one of nearly 60 leaders who met in Vienna for a summit on relations between the EU and Latin-American and Caribbean countries. Tony Blair was also at the meeting but the two did not meet in London.

Despite Venezuela's repeated assertions that it works against terrorism, and particularly militants in the Andean region, Washington accused it on Monday of being uncooperative in the U.S. war on terrorism.

The arms ban symbolically escalates a diplomatic crisis between Washington and oil-rich Venezuela and comes after years of friction between the two nations on issues ranging from trade to oil prices.

Chavez has spent the past few days in London meeting various figures from the British political left. He has not met British Prime Minister Tony Blair, whom he once criticised as "the main ally of Hitler" for his close ties with President Bush.

From London, Chavez was due to travel to Algeria, a close ally and fellow OPEC member, and then to Tripoli for talks with Libyan leader Muammar Gaddafi on Tuesday.

The reporter then cited Chavez's critique of a previous question as "silly" for having motivated her question.

That original question, from CNN's Robin Oakley, asked whose decision it had been for Chavez not to include a visit with Prime Minister Tony Blair on his itinerary. During Chavez's first visit as head of state, five years ago, he was warmly received by Downing Street and Buckingham Palace.

Chavez derided the question as "silly" because, he said, the current visit is a private one, not a state visit.

He said he includes people with whom he disagrees among his friends, and cited Colombian President Alvaro Uribe, a conservative, as one of them.

Separately, Chavez warned that an attack against Iran would cost the world's oil consumers dearly.

"If there was an attack against Iran, the price could go to $100" per barrel from the current level of about $70, he said.

"It will lead also to greater destabilization."

Published reports have said the United States has drawn up plans to attack Iran if Tehran fails to abandon its nuclear program. U.S. officials have said there are no specific plans to do so.

No country, Chavez said, "has the right the prohibit a country from having nuclear energy." He said he is sure that the Iranians are not working on a nuclear weapon, as U.S. officials have claimed.

"The Iranians, like us, want peace," he said.

Either way, he predicted, the energy crisis will deepen. He described capitalism as "extreme individualism," which is using up the world's non-renewable energy reserves at an alarming pace.

He said the twin towers of the World Trade Center consumed more energy than do some entire countries in Africa.

In addition, the fact that 90 percent of vehicles carry no more than one person is "a stupid thing," he said.

"Our planet will not put up with this," he said. "We're all in peril."

On Saturday, Chavez said he wanted to provide cheap heating oil for low-income Europeans.

The Venezuelan leader worked out a similar deal to deliver discount heating oil this past winter to needy Americans in parts of the eastern United States.

"I'd like to do the same here in Europe," he said Saturday evening at a gathering in Vienna of activists and representatives of social movements and non-governmental groups, according to Reuters. (Full story)

In Washington, the U.S. State Department on Monday added Venezuela to the list of countries "not fully cooperating" with counterterrorism efforts.

Venezuelan President Hugo Chavez has warned the US that any attack on Iran will have devastating consequences and send oil prices soaring.

Mr Chavez, on a two-day trip to the UK, called for a socialist new world order and said nations were cowards for not standing up to the "American empire".

The US has not ruled out military action against Iran over its nuclear programme but is pursuing diplomacy.

Hundreds of supporters gave Mr Chavez a rapturous welcome in north London.

Packed ballroom

The president told the meeting in Camden, hosted by Mayor of London Ken Livingstone, that a US military attack on Iran would lead to Tehran cutting off its oil supply.

"If the United States attacks Iran... oil could reach $100 (£52) a barrel or more," he said.

"Moreover, Iran has said it would attack Israel, and I know they have the wherewithal to do so.

"This would be a terrible escalation and I do not know where it would end and I do not know who would get out the first nuclear bomb or how many people would die. No-one would be safe from this madness."

Mr Chavez said the US "doesn't know what to do" in Iraq, which he called "the Vietnam of the 21st Century".

Jubilant supporters cheered him, many clad in Venezuela's red, blue and yellow and banging drums, while hundreds more were left outside the packed ballroom.

Mr Chavez said capitalism was a "destructive" system and that "socialism is the way forward".

Mr Chavez called unrestricted free trade "a trap by the world's most powerful so they can keep the weakest in slavery".

Britain and a number of other countries have criticised Mr Chavez's moves to exert greater control over his country's oil reserves - the world's fifth largest.

On Friday Prime Minister Tony Blair urged Venezuela to use its energy resources responsibly.

The president's trip has been described as a private one - he has made no request to see Mr Blair or any government officials.

On Monday Mr Chavez will have lunch with Mr Livingstone, and will deliver a lecture in the evening.

BBC Americas analyst James Painter says Mr Chavez's strategy for this trip is similar to the one he used when visiting the US in September last year. He does not meet government officials but rather appeals over their heads to appear as a man of the people.

He says Mr Chavez does not want a "European flank" opening that would support the critical comments made of his regime by the US.