Three international air carriers have grounded flights from Europe to the United States amid reports that al-Qaida could be planning a terrorist attack on a passenger plane.

British Airways, Air France and Continental Airlines canceled six flights for Sunday and Monday, citing security concerns. Return flights were also canceled.

British Airways and Air France each canceled two Washington-bound flights. The British carrier also scrubbed a flight from London to Miami, Florida. In addition, U.S.-based Continental Airlines grounded its flight today from Glasgow, Scotland, to Los Angeles.

The U.S. Department of Homeland Security said it had shared new intelligence with Britain, France and other foreign governments about a possible al-Qaida plot.

The Washington Post says the possible threats included releasing a deadly biological agent like smallpox or anthrax on to the aircraft, or transporting a radiological device.

Despite the cancellations, the Department of Homeland Security says it has no plans to raise the nations terror alert level.

The United States raised its terror threat level to "high" (orange) in late December, and terrorism concerns led to international flight cancellations and delays by British Airways, Air France and other airlines. The threat level was lowered to "elevated" (yellow) early on January 9.

Some information for this report provided by AP, AFP and Reuters.