The US government has raised its terror alert to the second-highest level and warned there was a high risk of attacks that could be bigger than those of September 11th, 2001.
Homeland Security Secretary Mr Tom Ridge said threat indicators are "perhaps greater now than at any point" since the 2001 attacks and promised that all federal departments and agencies would increase their defences against what he called "al-Qaeda's continued desire to carry out attacks against our homeland".
The warning came despite White House assurances that many of al-Qaeda's operations had been disrupted and that the occupation of Iraq was making the world safer.
Mr Ridge said al-Qaeda might try to use aircraft in new attacks - as they did to strike the World Trade Center and the Pentagon more than two years ago, killing nearly 3,000 people.
US officials attributed the warning to "credible sources" and ordered the color-coded alert system raised to orange - denoting "a high risk" of terrorist attacks - from yellow, which the Department of Homeland Security defines as "a significant" or "elevated" risk of terrorist attacks.
It is the fifth time the orange alert has been activated since the system was instituted in March last year.
"The information we have indicates that extremists abroad are anticipating near-term attacks that they believe will either rival or exceed the attacks that occurred in New York and the Pentagon and the fields of Pennsylvania," said Mr Ridge, who met yesterday with President George Bush's homeland security advisers.
Mr Ridge said security would be increased at the nation's airports and more agents would be deployed along its borders. Coast Guard air and sea patrols will also increase to protect the nation's critical ports and shipping lanes, he added.
US officials singled out New York and Washington as possibly the highest-profile targets.