(Reuters) -A United Nations expert said on Monday that U.S. government treatment of Guantanamo Bay inmates was cruel, inhuman and degrading under international law and called for Washington to apologize and provide reparation.

"I observed that after two decades of custody, the suffering of those detained is profound, and it's ongoing," Fionnuala Ni Aolain said at the United Nations after completing the first official visit by a U.N. expert to the detention facility in Cuba.

"Every single detainee I met with lives with the unrelenting harms that follow from systematic practices of rendition, torture and arbitrary detention," she said, citing what she called the undue use of restraints and near constant surveillance as current shortcomings.

The Pentagon did not immediately respond to a request for comment.

The prison was set up in 2002 by then-U.S. President George W. Bush to house foreign militant suspects following the 9/11 attacks on the United States. Its population grew to a peak of about 800 inmates before it started to shrink.

President Joe Biden has said he wants to close the facility but has yet to present a plan to do so, and some 30 prisoners remain. The comments by the independent expert add to recent criticism from the Red Cross and another UN body.

"The U.S. government must urgently provide judicial resolution, apology and guarantees of non-repetition," she added. She praised Washington for the access she was granted. "Few states exhibit that kind of courage," she said.

(Reporting by Emma Farge; Additional reporting by Idrees Ali in Washington; Editing by Chris Reese and Howard Goller)