The world has reached a "critical moment" in terms of the "magnitude of the threat" terrorists now pose, said Donald Rumsfeld, US defence secretary, who warned that the fight against terrorism would be a "marathon" and not a sprint.

Addressing a press conference, Mr Rumsfeld said that terrorists could already have or obtain weapons of mass destruction.

"Almost anything that people want, they can get their hands on, if they're determined and if they have the money," Rumsfeld said.

Mr Rumsfeld confirmed that the US had begun to move forces to prepare for military action following the September 11 terrorist attacks on New York and Washington.

"What we're doing is trying to get ourselves arranged in the world with our forces in a way that makes them useful" if President George W Bush decides to deploy them, he said.

Mr Rumsfeld declined to offer details of the kind of troops or military craft were being deployed, or where. He could not confirm that the military campaign had been officially named "Operation Infinite Justice", saying that this label was "under review".

"If I can do anything today, I'd like to disabuse people from trying to draw parallels between past conflicts and this one."

According to reports, the military movements include the transfer from the US to the Gulf region of more than 100 fighter and bomber aircraft to join more than 200 that have been based in that region patrolling Iraq. AWACS radar surveillance planes and support aircraft are also thought to be transferring to the Gulf.

A naval battle group led by the aircraft carrier "USS Theodore Roosevelt" and featuring 14 ships including amphibious assault vessels left Norfolk, Virginia on Wednesday. Two other carrier battle groups are already in the Gulf region.

President George W. Bush is to address a joint session of Congress on Thursday evening to "explain to the American people who would do this to our country and why."