"We are exceptionally requesting a Presidential pardon for Mr. Abu Zubaydah, owing to his treatment while in detention and the lack of due process since he was first detained," a dozen independent UN experts said in a statement.

"His immediate release and relocation to a third safe country are long overdue."

Abu Zubaydah was the first of a number of prisoners to be subjected to CIA "enhanced interrogation" techniques following the September 11, 2001 attacks on the United States.

The Saudi-born Palestinian, whose full name is Zayn al-Abidin Muhammad Husayn, was captured in Pakistan in 2002 and has been held without trial at the US Guantanamo camp in Cuba since 2006.

He was waterboarded 83 times and suffered other physical abuse, according to a US Senate report, which said that the CIA conceded he was never a member of Al-Qaeda and not involved in planning the 9/11 attacks.

The UN experts, including the special rapporteurs on torture and on promoting human rights while countering terrorism, warned in Wednesday's statement that Zubaydah "suffers serious health conditions".

Those included "injuries sustained during torture that are allegedly exacerbated by the denial of medical attention", they said.

The experts, who were appointed by the UN Human Rights Council but who do not speak on behalf of the United Nations, also lamented that his "lawyer-client communication has been seriously impeded".

They highlighted findings by a range of international and regional rights mechanisms that Zubaydah suffered multiple violations linked to the US rendition and secret detention programme.

Zubaydah had endured "profound psychological and physical trauma of torture and other cruel, inhuman and degrading treatment and enforced disappearance", they charged, demanding he be granted compensations and reparations.

He is among 15 people still being held at the controversial American base, after the US Defence Department on Monday said it had resettled 11 Yemeni detainees from Guantanamo to Oman.

Biden pledged before his election in 2020 to try to shut down Guantanamo, but it remains open with just weeks left in his term.

The facility was opened in the wake of 9/11 and has been used to indefinitely hold detainees seized during the wars and other operations that followed. Some 780 prisonners have spent time there.

The conditions there and the denial of basic legal principles have prompted consistent outcry from rights groups, and UN experts have condemned it as a site of "unparallelled notoriety."