A prisoner has been found dead at the Guantanamo Bay naval base, two days after he was apparently found unconscious in his cell at the isolated, high-security prison, the US military has said.

The unidentified prisoner’s name and nationality were not disclosed pending notification of family, military officials said on Tuesday.

The detainee was found "unconscious and unresponsive" by guards during a routine check, the US Southern Command said.

"After extensive lifesaving measures had been performed, the detainee was pronounced dead by a physician," it said in a statement.

The prisoner was the ninth detainee to die at the facility since it was opened in January 2002 to hold men suspected of terrorism or links to al-Qaeda and the Taliban.

Of the 779 men held there, 167 remain.

A medical examiner was sent to the base to determine the exact cause of death, and an investigation will be conducted by the Naval Criminal Investigative Service.

His remains will be returned home after the autopsy.

'Disciplinary status'

The man died in Camp 5, a maximum security facility that holds those accused of breaking camp rules. He was "in disciplinary status" after recently assaulting a guard with a "cocktail" of bodily fluids, a military official said.

Camp Five is a facility that "houses those detainees deemed to be the highest threat to themselves, other detainees or guards", said Jose Ruiz, a spokesman for the Miami-based Southern Command.

Two of the earlier deaths were from natural causes and six were designated as suicides, most of them by hanging.

The dead man was not among the small group currently facing charges in the war crimes tribunal at the Guantanamo base in eastern Cuba, and he had not been designated as eligible for prosecution, said Navy Captain Robert Durand, a spokesman for the detention operation.

About half of the Guantanamo detainees have been cleared for release, but it was not known if he was among that group.

Most of the prisoners currently at Guantanamo are housed in Camp 6, a facility that allows communal living arrangements among the inmates and is designed for less intensive monitoring by guards.

The prisoner had previously been a hunger striker whose weight dropped low enough that he was force-fed nutritional supplements.