Soldiers patrol city streets as Americans are warned of imminent terrorist threat

The red and white of Father Christmas mingled with the black steel of semi-automatic gun barrels on Fifth Avenue yesterday as New York deployed heavily armed police officers at strategic locations in the face of a newly heightened threat of terror attacks.

Officials across the United States responded to the decision on Sunday from the Department of Homeland Security to elevate the national terror alert level to orange, the second highest level. It was an indication of serious concern in the intelligence community that al-Qa'ida was preparing to strike again.

Police and National Guard soldiers were patrolling airports, railway stations, bridges and other busy landmarks in most large American cities. It is the fifth time that America has been put on orange alert since the colour-coded system was introduced after the 11 September attacks in 2001.

Tom Ridge, the director of Homeland Security, repeated his message of Sunday that the country should be especially vigilant. He said the warning was based on information which indicated that attacks might be imminent. He did not specify the sources of the information or where the attacks could occur. Officials said that there was particular concern that terrorists might use aircraft again.

Mr Ridge said: "The volume is up. The quality of the reporting is up. The credibility is there." He said the assaults could be more devastating than those of 11 September. Referring to the intelligence, he said: "We've never quite seen it at this level before."

Mr Ridge called on Americans to be extra vigilant but added: "If you've got holiday plans, go. I think it's very important to send a message to the terrorists of goodwill and resolve."

Worries about a terrorist attack have been mounting for some time. Last week, federal officials warned New York and other large cities about the increased risks of attacks during the holiday season. The decision to raise the alert level to orange for the first time since May might in part have been prompted by the broadcast by the Arabic television network al-Jazeera on Friday of a new tape recording allegedly made by Ayman al-Zawahiri, Osama bin Laden's deputy. The voice on the tape said al-Qa'ida was "still chasing Americans and their allies everywhere, even in their homeland".

The State Department also issued an alert on Sunday, warning Americans abroad to increase their vigilance, especially in public places such as restaurants, hotels and places of worship.

The suggestion of attacks on domestic soil causes most nervousness - "I'm from Maine," said Daniel Bennett who was visiting New York, "and this makes me glad of it". But with airports and stations packed with holiday travellers, there was no sign of Americans making major concessions to the warning. "They're like earthquakes. You learn to deal with it," said Jeff Shaw, of Reno, Nevada, at the San Francisco Shopping Mall. "If it's going to happen, it's going to happen."