Fighter jets are also patrolling the skies around the United States capital after the Pentagon activated increased security.

SAFEGUARDS

In Washington: Anti-aircraft missiles F-16 fighter jets and helicopters Radar In London: 1,500 police and soldiers guarding main airport Roads along flight paths also secured

In Britain, 1,500 armed troops and police were deployed to protect London's Heathrow airport which ministers believe could be a target.

The action follows the release of the latest message said to be from Osama Bin Laden, which called for armed opposition to any attack on Iraq, which could be led by the US and UK.

Intelligence agencies on both sides of the Atlantic have warned they believe an attack - possibly by Bin Laden's al-Qaeda network - could happen within days.

'Specific threat'

The US authorities have indicated they are more worried about an attack than at any time since passenger planes were hijacked and flown into buildings in New York and Washington on 11 September 2001.

Whether this is a signal of impending attack or not is something we're looking at George Tenet CIA Director

CIA Director George Tenet told a Senate committee on Tuesday: "This is the most specific [threat] we have seen."

On Wednesday, he told the same committee that broadcasts of statements believed to be from the al-Qaeda leader often preceded attacks.

He said a tape played on 6 October last year came before an attack on a French tanker in Yemen, the killing of a US diplomat in Jordan and the Bali nightclub bombing.

Another message released on 12 November was followed by an attack on Israeli targets in Kenya, he said.

Mr Tenet said it appeared Bin Laden wanted to encourage his followers.

"He is obviously exhorting them to do more," he said.

"Whether this is a signal of impending attack or not is something we're looking at."

The UK has increased security visibly at Heathrow

A spokeswoman at the Pentagon said the batteries of Stinger anti-aircraft missiles had been activated following assessment of new threats.

F-16 fighter jets and helicopters are also being used to protect the city and radar is being monitored.

The Pentagon building - one of the targets on 11 September - is being covered as are other landmarks.

Americans have been following advice to make emergency plans and have been stocking up on supplies of food and water.

British action

A senior member of the UK Government, John Reid, referred to the 11 September attacks when discussing what danger was being faced in London and other cities.

"This is about a threat of the nature that massacred thousands of people in New York," he said.

But Home Secretary David Blunkett said it was not necessary to close Heathrow - one of the world's busiest airports.

He told the public to be vigilant but not frightened and said he hoped the country would get through the next few days without incident.

Bin Laden's message

The possibility of an attack in Britain is being linked to the Muslim religious festival of Eid al-Adha, which coincides with the end of the annual Hajj pilgrimage which runs until Saturday.

The heightened security also comes as the United Nations prepares to hear a new report on Iraq from weapons inspectors on Friday.

We stress the importance of martyrdom attacks against the enemy; these attacks inflicted on America and Israel a disaster they have never experienced before Voice identified as Osama Bin Laden

Click here for Bin Laden's message

The US and the UK have been the leading international voices saying an invasion may be the only way to ensure Iraq has no banned weapons.

In a message broadcast on Tuesday, a man believed to be Bin Laden called for armed resistance including suicide missions to protect Iraq.

The taped statement has raised new tensions about any action to be taken against Iraq.

The US says it is further evidence of a link between Iraq and al-Qaeda while Germany says there is no proof. Iraq has also denied being a backer of Bin Laden.