Human Rights Watch, a group based in New York, said in a report on Monday that detainees were kept in total darkness and were tortured and mistreated by American and Afghan guards in civilian clothes.
The facility, called by the inmates the "Dark Prison", was run near Kabul, the capital of Afghanistan, Human Rights Watch said in its report, which was based on the testimony of several detainees at Guantanamo Bay, Cuba, who said they were held there.
According to the report: "Some detainees said they were shackled in a manner that made it impossible to lie down or sleep, with restraints that caused their hands and wrists to swell up or bruise.
"They were chained to walls, deprived of food and drinking water, and kept in total darkness with loud rap, heavy metal music, or other sounds blared for weeks at a time."
CIA officials have not commented on various allegations of torture, but Dick Cheney, the US vice-president, denied on Sunday that America engaged in torture.
"I can say that we, in fact, are consistent with the commitments of the United States that we don't engage in torture, and we don't," Cheney said in an interview to be broadcast on Monday on ABC News Nightline.
Cheney was not responding directly to the Human Rights Watch report, but to questions about anti-torture legislation before Congress.
Cheney denies that the US
engages in torture
Human Rights Watch did not speak with the inmates directly because the United States has not allowed human rights organisations to visit detainees at Guantanamo or other detention sites abroad.
Instead, the testimony regarding the alleged prison was made by the detainees to their lawyers, who passed it to the rights watchdog.
"Human Rights Watch believes that the detainees' allegations are sufficiently credible to warrant an official investigation," the report said.
John Sifton, terrorism and counterterrorism researcher at Human Rights Watch, said: "We're not talking about torture in the abstract, but the real thing. US personnel and officials may be criminally liable, and a special prosecutor is needed to investigate."
The report said Benyam Mohammad, an Ethiopian-born Guantanamo detainee who grew up in Britain, claimed that he was held at the facility in 2004.
"It was pitch black, no lights on in the rooms for most of the time," he was quoted as telling his lawyer.
"The CIA worked on people, including me, day and night"
Benyam Mohammad, an Ethiopian-born Guantanamo detainee
"They hung me up. I was allowed a few hours of sleep on the second day, then hung up again, this time for two days."
He went on to say that he was forced to listen to Eminem and Dr Dre for 20 days before the music was replaced by "horrible ghost laughter and Hallowe'en sounds".
"The CIA worked on people, including me, day and night," he was quoted as saying.
"Plenty lost their minds. I could hear people knocking their heads against the walls and the doors, screaming their heads off."
The report said the prison was closed after several detainees were transferred to a US military detention centre near Bagram, just north of Kabul, late last year.
America's handling of its detainees has come under increasing scrutiny in recent weeks.
Khaled al-Masri, a German citizen of Lebanese descent, is suing the CIA for wrongful imprisonment and torture, saying he was seized in Macedonia on 31 December 2003, and taken by CIA agents to Afghanistan, where he was allegedly abused before being released in Albania in May 2004.
Senior members of the European Parliament, meanwhile, have proposed setting up an investigative committee to determine whether US agents held terror suspects in secret European prisons.