The US law enforcement authorities will start detaining dozens of suspected sympathisers of President Saddam Hussein in at least five US cities, even as FBI issued an alert warning of possible strikes by Al Qaeda terror network in which they could use chemical, biological or radiological weapons.

"The US intelligence community believes that terrorists will attempt multiple attacks against US and coalition targets worldwide in the event of a US-led military campaign against Saddam Hussein," the FBI alert said.

"Al Qaeda may be in the last stages of planning for large-scale attacks. There are many recent indications this planning includes the use of chemical, biological, and/or radiological materials," it added.

Iraqi agents are also "inconsistently competent" at a variety of terrorist tactics, the alert said, including "assassinations, armed assaults, hijackings, kidnappings and bombings."

"Al Qaeda's interest in cyanide and other poisonous gases, and reporting that the group would use them in enclosed spaces, suggests the group may attempt to employ such weapons in buildings or in aircraft or in subway or railway cars," the bulletin added.

According to the alert, the Iraqi agents have the expertise to "construct sophisticated improvised explosive devices," ABC television network reported.

The alert comes as the nation is put on high alert and law enforcement officers are out in full strength to ensure that nothing untoward happens.