The CIA's secret interrogation of the 14 "high-value detainees" it is holding amounted to torture, the International Committee of the Red Cross says.
The charge is contained in published excerpts of the ICRC's 2006 internal report due to appear in the April 9 issue of the New York Review of Books.
The ICRC report was obtained by Mark Danner, a journalist and professor at the University of California at Berkeley.
The 14 prisoners were visited by the ICRC after they were moved from secret interrogation sites and prisons to Guantanamo Bay naval base in 2006.
Private access to the detainees, whose nationalities are not given, was granted by George Bush, the former US president.
The neutral, Swiss-based ICRC is designated by the Geneva Conventions on warfare to visit prisoners of war to ensure countries respect their obligations under the 1949 accords.
ICRC officials would not confirm details of the report to the media and denied leaking it.
"We regret that information attributed to the ICRC has been made public," said Simon Schorno, a spokesman for the ICRC.
"In addition, many other elements of the ill-treatment, either singly or in combination, constituted cruel, inhuman or degrading treatment"
International Committee of the Red Cross internal report
"We share our observations and concerns related to US detentions as part of the confidential dialogue we maintain with US authorities and so we do not wish to comment on the substance of the article."
The ICRC generally refuses to comment on its prisoner-of-war investigations, reasoning that it undermines the organisation's ability to gain access to the prisoners and influence how they are treated.
A US official familiar with the ICRC report noted that the claims of abuse were made by the alleged terrorists themselves.
The official asked to speak anonymously because the CIA interrogation programme is classified.
A purported al-Qaeda training manual, obtained by police in Manchester, UK in 2000 from the computer of an alleged al-Qaeda operative, instructs adherents to claim torture or abuse if they are captured.
The document was translated and posted onto the US justice department's website.
According to the ICRC report, as described by Danner, the prisoners separately and consistently described long-term solitary confinement and waterboarding, which simulates drowning.
They were also subjected to prolonged stress positions, forced prolonged nudity, beatings, other forms of abuse and denied solid food.
"The allegations of ill-treatment of the detainees indicate that, in many cases, the ill-treatment to which they were subjected while held in the CIA programme, either singly or in combination, constituted torture," states the report, according Danner.
"In addition, many other elements of the ill-treatment, either singly or in combination, constituted cruel, inhuman or degrading treatment."
The report was written shortly after Bush publicly declared that the US had not tortured detainees at secret CIA prisons known as "black sites".
Barack Obama, the US president, has ordered the sites closed and has restricted the CIA to using only those interrogation methods approved for use by the US military until a complete review of the programme is conducted.
A leaked 2003 ICRC report said that prisoners in US custody in Iraq had been abused in ways that in some cases was tantamount to torture.
The report was written before the abuses at the Abu Ghraib prison became public. It was leaked shortly after they became known.