A Moroccan prisoner released from Guantanamo in 2004 was Osama bin Laden's top bodyguard, according to testimony on Thursday in the war crimes trial of another alleged bodyguard.

Defendant Salim Hamdan had said the Moroccan, Abdellah Tabarak, was head of the al-Qaeda chief's security detail, said US Naval Criminal Investigative Service agent Michael St Ours, relating what Hamdan told him in a 2002 interrogation.

St Ours, who also questioned Tabarak at Guantanamo, said he was surprised to learn from Hamdan's lawyers that the Moroccan had been released because he was a "very hard individual".

Tabarak was one of five Guantanamo prisoners returned to Morocco in August 2004. He was then suspected of having served as a bodyguard for the al-Qaeda leader in Sudan and Afghanistan.

US Army Colonel Steve David, chief defence counsel for the Guantanamo tribunals, said several men suspected of more serious crimes than Hamdan have been released from the military prison at the US Navy base in southeast Cuba.

"That's the nature of the beast," he said. "Do we like it? No."

The US military does not comment on decisions to release individual detainees.

Hamdan, a Yemeni, has been held at Guantanamo since May 2002. He is accused of transporting weapons for al-Qaeda and helping bin Laden to escape US retribution after the September 11 attacks by helping him avoid being detected in Afghanistan. He faces a maximum life sentence if convicted of conspiracy and supporting terrorism.