A terrorist planning to detonate a nuclear device in the USA has been arrested by FBI agents.

The attack was foiled when he was arrested on arrival at Chicago's O'Hare airport.

US agents arrested an American whom they claim is an al-Qaida operative and was planning to detonate a radioactive 'dirty bomb,' Attorney General John Ashcroft said this afternoon.

The man, Abdullah Al Mujahir, was arrested on May 8.

"We have captured a known terrorist who was exploring a plan to build and explode a radiological dispersion device, or dirty bomb, in the United States,'' Ashcroft said in Moscow.

Ashcroft said the suspect was in the custody of the US military and being treated as an enemy combatant, suggesting plans for the first military trial of an alleged terrorist.