The United States has raised its terrorism alert from "elevated" to "high" and stepped up security around bridges, airports, nuclear power stations and industrial plants following threats of an Al Qaeda attack during the holiday season.

A warning was also issued to American citizens worldwide.

Homeland Security Secretary Tom Ridge said in Washington, DC, that the move was the result of a "substantial increase" in the volume of intelligence pointing to "near-term attacks that could either rival or exceed what we experienced on September 11".

The indicators showed that the threat level "perhaps is greater now than at any point" since September 11, 2001, officials said.

The Federal Bureau of Investigation sent out warning to thousands of state agencies as the officials said they considered New York, Washington and some cities on the west coast as high risk. They were especially concerned about bridges, tunnels, chemical plants, nuclear power plants, dams and high rise building.

Reports also hinted at the possibility that terrorists might try to use planes as weapons like they did on September 11.

"Recent reporting reiterates -- and this is a constant stream of reporting -- that Al Qaeda continues to consider using aircraft as a weapon," Ridge said. "And they are constantly evaluating procedures, both in the United States and elsewhere, to find gaps in our security posture that could be exploited."

"We have enhanced security at our nation's airports and around other transportation systems and infrastructure," he said.

"We will re-deploy agents and other resources at our borders to meet the current threat. There will be more Coast Guard air and sea patrols off our shores, in our ports and escorting ships."

In New York, Mayor Michael Bloomberg assured people that the city was fully prepared and promised to increase police presence at landmarks and financial institutions, increase check points at tunnels and bridges and enhance patrolling of waterways.