“The potential for us to be hit again is a very real potential,” Attorney-General John Ashcroft told Fox news, although he claimed dozens of possible attacks had been thwarted.

He added that steps had been taken to reduce the risk of an attack. The practice of allowing foreign transit air passengers to change planes in the US without a visa, he said, had been suspended.

Ashcroft was responding to an audio tape broadcast on Sunday that reportedly contained a message from Ayman al-Zawahiri, believed to be the second highest ranking official in al-Qaida.

The message, broadcast on Sunday by the Dubai-based television network Al-Arabiya, warned the US it would pay a high price if it harmed detainees at its naval base in Cuba. The “real battle” was still to come, it said.

‘Al-Qaida won’t wait to attack’

Ashcroft dismissed the apparent connection between US behaviour at the Guantanamo Bay base and a possible al-Qaida response, saying the group blamed for the September 11 attacks would act in any case.

“I believe al-Qaida wants to strike us. I believe they want to strike us whenever and wherever they can,” he insisted. “I believe we have disrupted dozens and dozens and dozens, over a hundred terrorist-related attacks around the world since 9/11,” he said.

Alleged al-Qaida tape threatens
revenge if detainees harmed

Also on Sunday, the secretary of the department of homeland security, Tom Ridge told CNN the measures against foreigners on transit flights was based on “a very specific piece of intelligence that talked about terrorists exploiting this particular vulnerability.”

Ridge added that the visa waiver program enjoyed by visitors from Western Europe and other friendly states would remain - but those travellers should expect closer scrutiny.

The US is holding more than 600 people from 42 countries without charge or legal counsel at its naval base at Guantanamo Bay in Cuba.

It announced recently it would begin to try selected prisoners at special military tribunals. Those convicted face the prospect of the death penalty.