Some US attack threats 'fabricated'

But they added that the dubious tip-offs were not the sole basis for raising the alert across the US last week to orange, the highest level since the anniversary of the 11 September attacks.

Separately, Homeland Security Secretary Tom Ridge said there was no need to panic, though Americans were wise to take precautions.

There has been mass buying of supplies by people preparing to create shelters for a radiological or chemical attack.

Security was increased at sites across the US after intelligence officials decided there was a clear danger.

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

Batteries of Stinger anti-aircraft missiles were set up around Washington and the capital's skies were patrolled by F-16 fighter jets and helicopters.

Lie-detector tests

The extra security is still in place despite the new concerns about the veracity of some information.

A senior administration official said terrorist suspects who had passed on news of the threats had been given lie-detector tests.

Authorities later decided the informants had probably fabricated their evidence, the official said on condition of anonymity.

Both Mr Ridge and President George W Bush stressed that business would carry on as usual in the capital and elsewhere.

Mr Bush said the raising of the "high-risk" orange alert was a "stark reminder of the era that we're in, that we're at war and the war goes on".

Mr Ridge said making preparations for an attack was sensible but he said there was no need "to start sealing the doors or windows".

Tight security remains in force at airports across the UK, where officials also had intelligence that the threat of an attack - possibly by al-Qaeda - had increased.