The Bush administration yesterday put the United States on a high state of alert, saying it had picked up credible intelligence of a possible terrorist attack "that could either rival or exceed" September 11th.
The Homeland Security Secretary, Mr Tom Ridge, did not give details of the expected nature of a potential attack, but mentioned "significant concern" that aircraft could be used again.
Homeland Security officials said there was also concern about the possible use of chemical, biological or radiological weapons.
It is the first time in seven months that the US has issued an orange alert - the second-highest status after red - and Mr Ridge said the threat was "perhaps greater now than at any point since September 11th, 2001".
He made the announcement, raising the alert status from yellow, after a "substantial increase" in the volume of "chatter" among suspected terrorists being picked up by US intelligence. "We continue to hear . . . the interest in using aircraft as a means of attack," he said.
The threat level has never reached the red alert. This would mean a terrorist attack is either imminent or under way.
US officials said it was impossible to tell whether the currently threatened attack would be aimed at US or allied interests abroad or against US territory, but Americans were warned that extra security precautions would be taken at airports around the country, lengthening holiday queues.
"Extensive and considerable protections have been, or soon will be, in place all across the country," Mr Ridge said at a news conference. "Your government will stand at the ready 24 hours a day, seven days a week, to stop terrorism during the holiday season and beyond." - (Guardian service)