Officials in the United States have tightened security at transportation and energy facilities following government warnings that al-Qaeda could strike over the holidays.

On Sunday, Washington raised the national security level from yellow to orange, which is "high" alert.

As a result, surface-to-air missiles have been deployed around Washington, D.C.

Also, more officers than usual are patrolling the U.S.-Canadian border, and maritime patrols are on duty near power plants on Lake Erie.

Similar defensive measures have been implemented around important U.S. bridges, tunnels, ports, landmarks and chemical facilities.

Security has been particularly tight at airports.

Tom Ridge

Travellers have been advised to arrive at airports early and expect delays.

When asked at a press conference to elaborate on the seriousness of the situation, U.S. Secretary of Defense Donald Rumsfeld said only that U.S. government officials had "good reason" to increase the national security level.

"Is it serious? You bet your life it is," he said.

Sunday's announcement by U.S. Homeland Security Secretary Tom Ridge marks the fifth time the U.S. has raised its security level to Code Orange since the Sept. 11, 2001, terrorist attacks.

Ridge briefed President George W. Bush about intelligence that he says indicates possible attacks even more devastating. "We have never quite seen it at this level before," he said.

Security was tightened up for this year's U.S. Thanksgiving weekend, so for many travellers the changes are par for the course.

But some security experts say there is a danger the public is no longer taking the warnings seriously. "Because you only go to the American public so many times before the public says 'You are crying wolf again. Nothing has happened. Why are you doing this?'" said former FBI agent Jack Cloonan.