New York: The United States has raised its nationwide terrorism alert from "elevated" to "high" and stepped up security around bridges, airports, nuclear power stations and industrial plants due to the threat of an al-Qaeda attack during the holiday season that could either "rival" or "exceed" the September 11 strikes.

Simultaneously, a warning to American citizens worldwide was issued.

Announcing the decision to raise the threat level, Homeland Security Secretary Tom Ridge said 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 Sept 11".

On the five-level colour coded scale, the threat level was raised, from yellow or elevated to orange, which stands for high risk of terrorists attacks. Yellow is the middle of the scale and orange second highest. The red is the highest, which would be used if threat is imminent or the attack is underway.

Officials say that the indicators show that the threat level "perhaps is greater now than at any point" since September 11, 2001.

Federal Bureau of Investigation (FBI) sent out warning to thousands of state agencies, as officials said they consider New York, Washington and some cities on west coast as high risk. They are especially concerned about bridges, tunnels, chemical plants, nuclear power plants, dams and high-rise buildings at high risk during the next two weeks.

PTI