About 380 prisoners are held at the US prison in Guantanamo Bay

A statement by the US Southern Command said the inmate was found unresponsive and not breathing by guards, and attempts to revive him failed.

Two Saudis and a Yemeni prisoner were found hanged in an apparent suicide at the camp in June last year.

There are about 380 prisoners at the camp, some held for five years.

'Appropriate care'

There were no details as to how the prisoner died. The Naval Criminal Investigative Service has begun an inquiry into the incident.

You have five-and-a-half years of desperation there with no legal way out

Michael Ratner

US Center for Constitutional Rights

"The detainee was found unresponsive and not breathing in his cell by guards," the statement said.

"The detainee was pronounced dead by a physician after all lifesaving measures had been exhausted."

A cultural adviser was working with the military to ensure that the prisoner's remains were handled "in a culturally sensitive and religiously appropriate manner", Southern Command said.

The president of the US Center for Constitutional Rights, Michael Ratner, told the Associated Press news agency the death was likely an act of desperation.

"You have five-and-a-half years of desperation there with no legal way out," Mr Ratner said.

'Unfair' trials

The death came just days before two detainees - Salim Ahmed Hamdan, a Yemeni, and Omar Khadr, a Canadian - were due to face trial before a US military tribunal on charges of war crimes.

On Wednesday, Mr Khadr fired his American lawyers, leaving him without representation for Monday's hearing.

Mr Khadr's former lawyer, Marine Lt Col Colby Vokey, said his former client was being held under a process that was "patently unfair".

"He doesn't trust American lawyers, and I don't particularly blame him," Lt Col Vokey said.

Mr Hamdan won a landmark case last year when the US Supreme Court ruled the military tribunal system illegal.

The decision forced US President George W Bush to return to Congress to authorise the tribunals.

Inmates at the Guantanamo Bay facility are not protected by the Geneva Conventions covering prisoners of war, the US says, as it describes them as "unlawful enemy combatants".