US terror warning over shopping centres

Terrorists plan to attack places "where large numbers of Americans shop", one of Osama bin Laden's three top aides has told US interrogators.

Abu Zubaydah also says that Bin Laden's al Qaeda network has been working on biological and chemical weapons.

The FBI has sent a warning to all its regional field offices in the US notifying them of a possible attack on shopping malls and they are liaising with local police on how to deal with the problem.

US officials do not know how seriously to take the information from Zubaydah, suspecting it could be empty boasting by a character they regard as a natural braggart.

He was the source of information that led the FBI to issue a warning a week ago that banks in the north-east of the US could also be targets. In Washington, a number of banks immediately shut their doors but there were no attacks.

Zubaydah also says that al Qaeda knows how to build a "dirty" bomb that could spread radioactive material over a large area. US intelligence has known for some time that al Qaeda would like to construct such a weapon, but is not sure whether to believe their informant when he says the knowledge and materials are already in al Qaeda's possession.

Dirty bombs use ordinary explosives to spray industrial-grade radioactive material in urban areas, with the aim of killing as many people as possible and making others ill, later to die from cancer.

Such attacks would paralyse commerce as whole cities would have to be shut down and decontaminated over extensive periods of time.

There is a strong desire on the part of American investigators to believe what Zubaydah is telling them.

The information they have been extracting from al Qaeda members of much lower rank at the US naval base in Guantanamo, Cuba, is thin and unsatisfying.

Not only is the information sparse, but its accuracy is in doubt, so that the pressure to obtain something credible from their prize prisoner is that much more intense.

Zubaydah was captured in Pakistan and turned over to US authorities last month.