Intelligence indicating that al-Qaida terrorists are seeking to release a chemical or biological agent aboard an airliner, or transport a radiological device in cargo, prompted the cancellation of six international flights scheduled for today and Monday, senior administration officials familiar with the reports said Saturday.

The use of such weapons would be a new tactic. The intelligence remains vague, and officials remain concerned about hijackings and other attack methods.

All the canceled flights are overseas flights arriving in the United States, as were the fights by foreign carriers canceled around Christmas. But on Saturday, for the first time, a flight by a U.S.-based carrier was canceled. Continental Airlines Flight 17, scheduled to fly today from Glasgow, Scotland, to Los Angeles with a stop in Newark, N.J., was canceled because the carrier was "unable to obtain the necessary security clearances from the Department of Homeland Security and their international counterparts," a Continental spokesman said.

British Airways canceled Flight 223 from Heathrow Airport in London to Dulles International Airport today and Monday, and Flight 207 from Heathrow to Miami today, after the airline was ordered to do so by the British government for "security reasons." A return flight from Dulles to Heathrow, Flight 222, also was canceled today and Monday. Flight 223 was canceled several times during the holiday season for security reasons.

Air France also cited security as the reason for canceling Flight 26 from Paris to Washington today and Monday. The return flight, Flight 27, which would have used the same airplane as Flight 26, also was canceled for both days.

Intelligence officials and others said the information about unconventional weapons doesn’t indicate a precise tactic. "There is some sketchy information that inserts (weapons of mass destruction) in connection with aviation," another intelligence official said.

Several intelligence officials said Saturday that al-Qaida appears desperate to mount a spectacular attack to show followers, new recruits and financial donors that it remains viable.

CIA officials say 75 percent of al-Qaida’s leadership has been killed or captured since the Sept. 11, 2001, terrorist attacks on the World Trade Center and the Pentagon.

There was no indication that today’s Super Bowl is threatened, but the Federal Aviation Administration imposed flight restrictions over Houston as part of stepped-up security.