The UN human rights chief, Navi Pillay, has urged the US to close Guantanamo Bay, saying the indefinite detention of many inmates there without charge or trial violates international law.
Ms Pillay said about half of 166 prisoners had been cleared to transfer either home or to third countries.
She also said she was "deeply disappointed" that the US was not honouring its pledge to close the camp.
It opened in 2002 to hold terrorism suspects after the 9/11 attacks.
President Barack Obama moved to close the facility at the US naval base in Cuba, but his plans to try suspects in US civilian courts were blocked by Congress.
"The continuing indefinite incarceration of many of the detainees amounts to arbitrary detention and is in clear breach of international law," Ms Pillay said in a statement on Friday.
"It severely undermines the United States' stance that it is an upholder of human rights.
"When other countries breach these standards, the US - quite rightly - strongly criticises them for it," she added.
Ms Pillay also said a continuing hunger strike among the inmates was a "desperate act" - but it was "scarcely surprising".
Human rights groups and lawyers representing the prisoners say the strike reflects growing frustration at the US military's failure to decide their future.
Many of those who have been cleared for transfer remain at the facility at the US naval base in Cuba because of Congressional restrictions and also concerns of possible mistreatment if they are sent back to their home countries.
Last month, Pentagon spokesman Maj Jeff Pool told the BBC that 31 detainees were on hunger strike.
He said that 11 of them were being fed liquid food through tubes and three were treated in hospital.