Airlines grounded a domestic US flight and several transatlantic flights during the weekend and yesterday amid reports US officials had intelligence suggesting al Qaeda may be planning a chemical or biological attack on an aircraft.

Three intelligence officials told the Washington Post the possible threats included releasing a biological agent like smallpox or anthrax on a plane so those aboard would spread the infection without knowing it.

British Airways, Air France and Continental Airlines said they had canceled several transatlantic flights scheduled for yesterday citing security concerns.

A grounded BA London-Washington flight was canceled several times in January because of security worries. ?There are a handful of flights we are concerned about, and British Airways has canceled about half of them,? a US official said, on condition of anonymity. ?We have received threat reporting that indicates al Qaeda?s desire to target these particular flights.?

Al Qaeda is held responsible for the September 11, 2001, attacks on the United States involving four hijacked commercial planes. About 3,000 people were killed in those attacks.

Intelligence value

US Homeland Security Secretary, Tom Ridge, has said the US government consistently receives intelligence that al Qaeda is still interested in using aircraft for attacks.

The Washington Post cited intelligence officials as saying attackers could try to hijack a plane by releasing a chemical agent to incapacitate the crew and passengers or smuggle a radiological device in luggage.

The newspaper said the intelligence on a possible attack with a weapon of mass destruction was vague.

US officials demanded enhanced security precautions that some airlines were unable to take, so ?it became easier to just cancel the flights,? an administration official told the Post.

Senator Jay Rockefeller, a senior Democrat on the Senate intelligence panel, said it was difficult to counter the threat of chemical or biological weapons smuggled onto planes.

?That?s partly the problem of not checking cargo, and it?s partly the problem of biological weapons, which nobody has figured out really what to do about yet,? he told Fox News. ?Nobody has any idea about what to do about them on an airplane or on the ground.?

Claudia Parsons