US security chiefs have warned of possible terrorist attacks in the US and Middle East towards the end of the week, including the use of poisons, chemicals and a device to spread radioactivity.

Director George Tenet, testifying to Congress in Washington, said intelligence information pointed to plots timed to coincide with the end of the Muslim religious festival of Eid, which runs from Wednesday to Saturday.

The intelligence is not idle chatter George Tenet, CIA Director

The new warning came as hundreds of soldiers were sent to guard London's Heathrow airport amid concern about al-Qaeda attacks in the UK capital.

Mr Tenet said the US had last week raised its threat level to "orange", the second-highest alert, because of highly specific information from sources with strong al-Qaeda ties.

"The intelligence is not idle chatter on the part of terrorists and their associates," Mr Tenet said.

"It is the most specific we have seen, and it is consistent with both our knowledge of al-Qaeda doctrine and our knowledge of plots this network - and, particularly, its senior leadership - has been working on for years."

"The information we have points to plots aimed at targets on two fronts - in the United States and on the Arabian peninsula."

'Iraq threat'

In a joint appearance before the Senate Intelligence Committee with FBI Director Robert Mueller, Mr Tenet said attackers might target places with light security, such as shopping malls or universities.

And he said al-Qaeda continued to have a presence in Afghanistan, as well as Pakistan, Iran and Iraq.

Mr Mueller said the FBI suspected there were "several hundred" Muslim extremists in the US who were working on fund-raising, recruitment and training.

"The enemies we face are resourceful, merciless and fanatically committed to inflicting massive damage on our homeland, which they regard as a bastion of evil," he said.

Both Mr Tenet and Mr Mueller said their agencies were far better prepared to detect and prevent attacks than they were before the 11 September 2001 attacks.

Heathrow patrols

In London, 450 troops have joined 1,000 extra police officers in patrolling the Heathrow airport as part of a tightening of security across the city.

Metropolitan Police commissioner Sir John Stevens said the decision had been taken in response to intelligence reports, but urged the public not to panic, only to be alert.

Scotland Yard said the Eid festival "may erroneously be used by al-Qaeda and associated networks to mount attacks".

But Muslim groups have criticised the police for making a link between the religious festival and a potential terror threat.

The London-based Islamic Human Rights Commission described the link being made by police between Eid and the terror threat as "unbelievable rubbish".