The national threat level is now orange, indicating a high risk of terrorist attack.
Homeland Security Chief Tom Ridge said the danger is “perhaps greater now than at any point” since September 11, 2001 – with strikes possible during the holidays.
He added that intelligence information gathered in the past few days suggests that extremists abroad are anticipating attacks that will rival or exceed the scope of September 11.
"Leaders in the private sector will be contacted," Ridge said. "All federal departments and agencies will meet the threat level - we're concerned about suicide bombings."
Ridge said officials did not see a connection between the capture of ousted Iraqi leader Saddam Hussein and the heightened security alert.
The threat information comes from multiple, credible sources but officials are unaware of a specific target or means of attack, a senior law enforcement official told The Associated Press.
Some of the intercepted communications and other intelligence mentions New York, Washington and unspecified cities on the West Coast, an official told The AP, who spoke on condition of anonymity. Authorities also are concerned about dams, bridges, nuclear plants, chemical facilities and other public works.