Treatment of detainees at Guantanamo Bay constitutes torture in some cases and violates international law, a leaked UN draft report says.
The report levels a raft of allegations against the US
The document, seen by the Los Angeles Times, suggests that investigators will recommend the prison camp is shut down.
It also questions the legal status of the camp and the classification of detainees as enemy combatants.
US state department spokesman Sean McCormack criticised the draft report as "hearsay".
The Los Angeles Times published the draft report on Monday and spoke to one of the authors, the UN special raporteur on torture, Manfred Novak.
"We very, very carefully considered all of the arguments posed by the US government. There are no conclusions that are easily drawn. But we concluded that the situation in several areas violates international law and conventions on human rights and torture," Mr Nowak told the LA Times.
The report suggests some of the treatment of detainees at Guantanamo Bay meets the definition of torture under the UN Convention Against Torture.
This includes the force-feeding of hunger strikers through nasal tubes and the simultaneous use of several interrogation techniques such as prolonged solitary confinement and exposure to extreme temperatures, noise and light.
The UN team also questions the legal status of the Guantanamo camp.
It says insufficient effort has been made to prove that the inmates really are enemy combatants.
It also recommends the prison camp is shut down.
"The US government should close Guantanamo Bay detention facilities without further delay," the report says. "The US government should either expeditiously bring all Guantanamo Bay detainees to trial ... or release them without further delay."
Mr Nowak was one of five UN envoys who interviewed former prisoners, detainees' lawyers and families during the past 18 months.
Investigators rejected an invitation to tour the base in Cuba because they would not have been allowed to interview the prisoners directly.
The US State Department has criticised the findings.
"Just because they decided not to take up the US government on the offer to go to Guantanamo Bay does not automatically give [them] the right to publish a report that is merely hearsay and not based on fact," spokesman Sean McCormack said.
UN officials will include responses from the US government before the report is officially released at the end of the week.
Mr Nowak says that nothing of substance will be altered when the final report is issued.
The investigation was ordered by the UN Commission on Human Rights.
The camp at Guantanamo Bay was set up in 2002 to hold foreign terror suspects, many of them captured in Afghanistan.
It currently houses about 500 suspects.