The US government has arrested an American citizen accused of conspiring with al Qaida terrorists to build and detonate a radioactive "dirty bomb" - with Washington as his target.

Attorney General John Ashcroft said that Abdullah Al Muhajir, a former Chicago street gang member who also goes by the name of Jose Padilla, was in the custody of the US military and was being treated as an enemy combatant.

A Justice Department official said that under legal rules, Muhajir can be held indefinitely as an enemy soldier.

A dirty bomb is a conventional explosive, mixed with radioactive material, which scatters when the bomb goes off.

There would not be a nuclear explosion, but the panic the bomb would cause would paralyse a city.

Mr Ashcroft said Muhajir, who converted to Islam, was arrested on May 8 as he flew from Pakistan into Chicago's O'Hare International Airport.

The 31-year-old is a native of New York City who moved to Chicago at the age of four.

"We have disrupted an unfolding terrorist plot to attack the United States by exploding a radioactive dirty bomb," said Mr Ashcroft.

He added that the government's suspicions about Muhajir's plans came from "multiple, independent, corroborating sources".

FBI Director Robert Mueller said: "Our principal interest is in preventing future terrorist attacks. This instance is an example of prevention."

Another senior administration official, speaking on condition of anonymity, said Muhajir was trained by al Qaida in Afghanistan and Pakistan to wire explosives and to research radioactive dispersal devices.

He was not believed to have had a bomb at the time of his apprehension.

"We have no information that suggests this advanced beyond planning stages," said White House Press secretary Ari Fleischer.

Deputy Defence Secretary Paul Wolfowitz said officials could not say with certainty that the US capital was the likely target, although Muhajir "did indicate knowledge of the Washington, DC area".

Muhajir was yesterday taken to a high-security US Navy prison in South Carolina, where he is being held separately from other prisoners.

* Three suspected suicide bombers have been arrested by Moroccan police in an operation to dismantle a terrorist cell with links to Osama bin Laden's network.

The men - Saudi nationals - were said to be planning suicide operations against British and US warships patrolling the Strait of Gilbraltar.

The names of the suspects were not released.