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 on Saturday they had canceled several transatlantic flights scheduled for Sunday and Monday citing security concerns.

Continental Airlines also scrapped a flight on Sunday from Washington to Houston, site of the Super Bowl football championship which draws one of the biggest TV audiences of the year in the United States.

A Homeland Security spokeswoman said the airline canceled Flight 1519 after the department recommended security measures in light of an unspecified threat. She said there was no intelligence suggesting a threat to the Super Bowl.

"The threat to this flight was similar to the type of information we had about the British Airways and Air France flights," the spokeswoman said, without elaborating.

A grounded BA London-Washington flight was the same service 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 U.S. 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.

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

INTELLIGENCE VAGUE

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.

U.S. 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."

Among the flights canceled was Continental's Sunday flight from Glasgow to Newark and an Air France flight from Paris to Washington Sunday and Monday. Sunday's BA207 flight from London to Miami was also canceled.

In December, Washington raised its terror alert to the second-highest level and asked British Airways and Air France to cancel several U.S.-bound flights.

U.S. authorities also asked foreign airlines to put armed marshals on some flights. Some EU countries oppose the sky marshals but France and Britain said they were ready to accept them in some cases.