Washington is breaking international law by holding detainees indefinitely at Guantanamo and must honour a pledge to shut the controversial jail, the UN's human rights chief said Friday.
"I am deeply disappointed that the US government has not been able to close Guantanamo Bay, despite repeatedly committing itself to do so," Navi Pillay said in a statement.
"The continuing indefinite incarceration of many of the detainees amounts to arbitrary detention and is in clear breach of international law," she said.
"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."
The jail, in a US Navy base in Cuba, was opened in 2002 to hold prisoners taken in the "War on Terror" waged by President George W. Bush after the 9/11 attacks.
His successor Barack Obama moved to close it in 2009, but plans to try suspects in US civilian courts were stymied by Congress, leaving many inmates in limbo.
"It is time to bring an end to this situation," said Pillay.
Dozens of detainees are staging a hunger strike which started in February amid claims -- denied by US officials -- that guards mishandled Korans during searches.
Its scale is disputed, with US officials putting the strikers at 37, while rights lawyers acting for detainees say a majority of the estimated 130 prisoners at Guantanamo's Camp 6 wing are involved.
"Given the uncertainty and anxieties surrounding their prolonged and apparently indefinite detention in Guantanamo, it is scarcely surprising that people's frustrations boil over and they resort to such desperate measures," said Pillay.
She said that half of the 166 remaining detainees at Guantanamo were declared to be "cleared for transfer" to their home countries or third countries, yet they remain imprisoned.
Others reportedly have been "designated for further indefinite detention," with some of them languishing in the facility for more than a decade, she said.
"No one is suggesting that the US should be 'soft' on people who have planned or carried out crimes or atrocities. Indeed, international law requires that there must not be impunity for such crimes," Pillay insisted.
But human rights apply to all, including those suspected of the most serious crimes, she underlined.