Prisoners are being abused at a "secret jail" in the main American military base in Afghanistan, according to a report from a US policy think tank.
Ex-detainees said they were deprived of sleep and held in cold isolation cells in the site at Bagram, says New York-based Open Society Foundations.
A BBC investigation in April uncovered similar allegations of prisoner abuse at a hidden facility in Bagram airbase.
The US military repeated its denial that it was operating a secret jail.
'Very troubling pattern'
Open Society Foundations, which is funded by liberal billionaire George Soros, says 18 detainees claim they were held at a secret site, dubbed the "black jail", during 2009 and 2010.
The inmates said they were exposed to excessive cold and light, not given enough food or blankets, deprived of sleep, stripped naked for medical examinations and not allowed to practise their religion.
"Given the consistency of the accounts, the Open Society Foundations believes these are genuine areas of concern, and not outliers, that run counter to US rules on detainee treatment," the report says.
"We're not talking about being threatened to death in interrogation with drills to their head, we're talking about run-of-the-mill detention conditions that when seen as a whole create a very troubling pattern," report author Jonathan Horowitz said.
The US military said its detention centres complied with US and international laws.
"The Department of Defence does not operate any secret prisons," Capt Pamela Kunze, a spokeswoman for the US military task force overseeing detentions in Afghanistan, told Associated Press news agency.
In April, the BBC spoke to nine former detainees who said they had been held at a secret jail.
The prisoners said that they had been prevented from sleeping and that a light had been kept on in their small, cold concrete cells so they could not tell if it was night or day.
In June last year, the BBC spoke to a number of other former detainees who had been held at Bagram airbase, and they claimed they had been beaten, deprived of sleep and threatened with dogs.