Thirty eight people believed to have been held in secret CIA prisons - or black sites - are missing, according to a report by a US human rights group.
The CIA allegedly flew secret detainees in unmarked jets
The Human Rights Watch (HRW) report also details torture allegations made by a terror suspect who was held in secret custody for more than two years.
The group has asked US President George W Bush to reveal the location of these detainees and close all US black sites.
Last year Mr Bush said the prisons had all closed and had not used torture.
In a televised address in September, Mr Bush admitted that 14 detainees had been held at secret CIA prisons that used interrogation methods that were "tough" but "lawful and necessary".
"The United States does not torture," Mr Bush said at the time. "It's against our laws, and it's against our values. I have not authorised it - and I will not authorise it."
He said the prisoners had since been transferred to the US military camp at Guantanamo Bay, Cuba, and the CIA was not holding any more terror suspects.
But in a report published on Tuesday, HRW has named another 38 people who were believed to have been held in secret CIA prisons, who are now missing.
Quoting US intelligence officials, The Washington Post says more than 60 people have been held in the prisons since 2001.
'Beaten and burned'
The group has called on the US to reveal the location of all detainees held by the CIA since 2001 and end its "illegal" secret detention and interrogation programmes.
Palestinian Islamic extremist Marwan al-Jabour told HRW he saw or spoke to a number of those named in the report while he was held by the CIA between 2004 and 2006.
John B Bellinger III has attacked EU probes into secret CIA flights
Mr Jabour, who was arrested in Lahore, Pakistan in May 2004, also detailed torture tactics he says were used against him while he was in US custody.
He says at various periods during his 28-month detention Pakistani authorities kept him naked and chained to a ceiling. He says he was beaten, burned and handcuffed in stress positions.
During this time he was also reportedly interrogated by US agents for hours on end, but Mr Jabour says he was only tortured when the Americans were not around.
Mr Jabour admits that in 1998 he trained in Afghanistan in the hope of fighting in Chechnya. He also says he helped Arab militants who had fled Afghanistan for Pakistan in 2003, but he denies any links to al-Qaeda or terror activities.
Meanwhile, the US has warned the European Union that ongoing inquiries into secret CIA flights within Europe linked to the black sites are threatening intelligence ties between Europe and the US.
The investigations "have not been helpful with respect to necessary co-operation between the United States and Europe," John Bellinger, legal adviser to US Secretary of State Condoleezza Rice, said.
Mr Bellinger also labelled a European Parliament report into the flights, released earlier this month, as "unbalanced, inaccurate and unfair".