The United States has raised its national security threat level for all commercial air flights and has barred passengers from carrying liquids aboard any planes.
U.S. officials say a code-red alert imposed early Thursday is aimed at commercial traffic from Britain, with focus on flights by United Airlines, American Airlines and Continental Airlines.
The liquids ban applies to all air travel in and out of the U.S. and includes beverages, hair gels and lotions. Baby formulas are exempt from the ban but will be examined. The threat level for all other commercial flights to or from the United States was increased to code orange, indicating a high risk of attack.
Homeland Security Director Michael Chertoff says there are no signs of plotting within the United States. But he said the plot uncovered in Britain was in its final planning stages.
Chertoff and FBI Director Robert Mueller said the plot has the earmarks of al-Qaida, but that no direct link has yet been established to the terrorist group.
Chertoff also said U.S. air marshals have been sent to Britain to boost security on flights to the United States.