Tom Ridge, the Director of Homeland Security, admitted yesterday that he suspected that there were active cells in the US, but he could not explain why none had been caught.

“I think we should assume and we should operate under the notion that some still are in the United States,” he said.

Security authorities in more than 60 countries have arrested suspects linked to Osama bin Laden, but none of the hundreds detained has yet been found to have any links with terrorism.

Only two of the top 20 most-wanted al-Qaeda suspects are known to have been caught or killed: Mohammed Atef, third-in-command to Osama bin Laden, died in a rocket attack on Kabul; and Anas al-Liby, a computer expert who lived in Manchester and is said to have helped to compile al-Qaeda’s terrorist training manual, has been arrested. No one in the American military or intelligence services knows where bin Laden is.

While the British and other governments accept that many of their young Muslims travelled to Afghanistan for training in al-Qaeda camps, the only American accused of following that path is John Walker, “the American Taleban” who is facing trial in Washington.

Mr Ridge marked the six-month anniversary of the terrorist attacks on New York and Washington by announcing that there would be a new national threat alarm system based on a five-level colour code.