Major US cities have tightened security after the nation's terror alert was raised amid intelligence warnings.
US officials have boosted security at San Francisco's Golden Gate Bridge
Militants appear determined to wreak "catastrophic destruction" on US targets, Homeland Security Secretary Tom Ridge told NBC television.
He said the change in the alert status - to orange - was the result of information from "many sources".
Major landmarks, airports, nuclear plants and shopping centres have all boosted security patrols.
"The information we have indicates that extremists abroad are anticipating near-term attacks that they believe will
either rival or exceed [the 11 September attacks]," Mr Ridge said in a statement on Sunday.
He said intelligence suggested that Osama Bin Laden's al-Qaeda network intended to use planes again as weapons.
The threat was "greater now than at any point since 11 September," he added.
"It seems that more of the comments, more of the observations and expectations from these folks is directed internally to the United States rather than interests abroad", he said, urging US holidaymakers to be "vigilant".
A senior US official has said New York, Washington and Los Angeles are al-Qaeda's main targets and that the risk of attack would be particularly high over Christmas and the New Year.
Major landmarks such as the New York Stock Exchange are being guarded.
Police patrols have been deployed to cover bridges, tunnels, major landmarks and "signature buildings" in New York City in response to the alert, New York Mayor Michael Bloomberg said.
US TERROR ALERTS
Red - severe risk
Orange - high risk
Yellow - significant risk
Blue - general risk
Green - low risk of attacks
Undercover police officers have been stationed around San Francisco's Golden Gate Bridge and mandatory truck stops have been introduced.
In addition to major cities and transport hubs, security has been increased at the US borders with Mexico and Canada.
The US Coast Guard has also boosted air and sea patrols to monitor US shipping lanes and ports.
The nation's five-level colour-coded alert system - introduced after the 11 September 2001 attacks - has been raised to orange, the second highest level.
Under orange alerts, federal agencies are urged to consider taking extra precautions at public events, to be ready to put contingency plans into operation and to restrict access to any threatened facilities.
Protective measures intended for red alerts include the closure of public and government buildings, closure of parts of the transport system and the pre-positioning of emergency teams and resources.
PREVIOUS ORANGE ALERTS
20 May 2003: after Saudi, Morocco bombings
17 Mar 2003: run-up to invasion of Iraq
7 Feb 2003: intelligence on increased al-Qaeda threat
10 Sep 2002: on first anniversary of 9/11 attacks
The BBC's Michael Buchanan in Washington says flights originating overseas and arriving in US airports are thought to be of particular concern under the current alert status.
But amid the warnings, Mr Ridge urged people to go ahead with their holiday plans.
"If we alter plans to go to celebrations then they have won... simply by threatening us," he said.
The dollar dropped to the latest
in a series of record lows against the euro on Monday following the attack warning.