London, Feb. 1 (Reuters): Western airlines grounded several flights to and from the US today amid reports US officials had intelligence suggesting al Qaida 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 yesterday they had cancelled several transatlantic flights scheduled for today and tomorrow citing security concerns.

A grounded BA London-Washington flight was the same service cancelled several times in January due to security worries.

“There are a handful of flights we are concerned about, and British Airways has cancelled about half of them,” a US official said, on condition of anonymity. “We have received threat reporting that indicates al Qaida’s desire to target these particular flights.” Al Qaida is held responsible for the September 11, 2001, attacks on the US involving four hijacked commercial planes. About 3,000 people were killed in those attacks.

US homeland security secretary Tom Ridge has said the government consistently receives intelligence that al Qaida 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 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.