The US government warned last night that the nation faced the threat of a wave of terror attacks over Christmas greater than at any time since the September 11 outrages.

Homeland Security Secretary Tom Ridge said: "Information indicates that extremists abroad are anticipating near-term attacks that they believe will rival or exceed the scope and impact of those we experienced in New York, at the Pentagon, and in Pennsylvania more than two years ago."

He cited reports that Osama bin Laden's terrorist network is trying to find holes in US aviation security.

The government elevated the threat level one notch to orange, the second highest state of alert after US intelligence agencies "received a substantial increase in the volume of threat-related intelligence reports".

"These credible sources suggest the possibility of attacks against the homeland around the holiday season and beyond," said Mr Ridge.

"These strategic indicators, including al Qaida's continued desire to carry out attacks against our homeland, are perhaps greater now than at any point since September 11."

A senior intelligence official said last week that analysts were particularly concerned about the threat of September 11-style attacks, in which terrorists would use hijacked airliners as weapons.

He added that "recent reporting reiterates that al Qaida continues to consider using aircraft as a weapon. They are evaluating procedures both here and abroad to find gaps in our security posture that can be exploited."

US officials told holiday travellers to be vigilant about the threat of terrorist attacks at the end of last week.

The warning was prompted, in part, by a raised level of ominous intercepted communications that has not lessened for months.

On Friday, the Arabic television network Al-Jazeera aired a new statement from Ayman al-Zawahri, the chief deputy of Osama bin Laden. The CIA believes the tape is authentic.

"We are still chasing the Americans and their allies everywhere, even in their homeland," said the voice on the tape.

Last night's alert shows that the US is not dropping its guard despite the capture of Saddam Hussein, which is already reaping dividends.

US troops rounded up suspected rebels yesterday during house-to-house searches in strongholds of the former Iraqi dictator, capitalising on intelligence gleaned from his capture. Troops killed at least two civilians in the operations.

General Richard Myers, chairman of US Joint Chiefs of Staff, said that several hundred Saddam loyalists had been held in recent raids, including "some of the leadership of this insurgency, absolutely, some of the cell leaders".

Gen Myers tied the arrests to the capture of Saddam, saying: "Some of the information we gleaned when we picked up Saddam Hussein led to a better understanding of the structure of the resistance from the former regime elements."

Saddam was arrested on December 13 in a hideout near his hometown of Tikrit, and the US military has said soldiers also seized a briefcase containing documents that shed light on the anti-US insurgency. The CIA is interrogating him in Iraq.

"The only word I have is that he's not being co-operative. But other than that, I don't know," said Gen Myers.