A human rights group says documents found in the Libyan capital, Tripoli, indicate the US and British spy agencies helped former leader Muammar Gaddafi persecute Libyan dissidents.
The documents were uncovered by the Human Rights Watch in the abandoned offices of Libya's former spy chief and foreign minister, Moussa Koussa.
The group said it uncovered hundreds of letters between the US Central Intelligence Agency, its British counterpart MI6 and Mr Koussa, who is now in exile in London.
The files shed new light on the US practice known as rendition during the so-called war on terror, in which terrorism suspects were handed over to other countries for interrogation.
Human Rights Watch says the papers indicate that several suspected militants who were abducted by the CIA were handed over to agents in Tripoli between 2002 and 2004.
Among them was provisional government official Abdel Hakim Belhadj, according to the documents.
Human Rights Watch says Mr Belhadj, now the military commander for Tripoli in the provisional government, was captured by the CIA in Asia and put on a secret flight back to Libya.
Mr Belhadj has said that he was tortured by CIA agents before being transferred to Libya, where he says he was then tortured at Tripoli's notorious Abu Salim prison.
Britain's MI6 was also said to have given details of dissidents to the Gaddafi regime.
Human Rights Watch says the papers outline the handing over of at least eight suspects to Libyan agents.
The group says the CIA suggested questions for Libyan intelligence officers to ask and that American agents were present during some interrogations.
In Washington, a CIA spokeswoman did not comment on any specific allegation but said that it could not come as a surprise that the agency works with foreign governments to help protect the US from terrorism and other deadly threats.
Western intelligence services began cooperating with Libya after Colonel Gaddafi abandoned his programme to build unconventional weapons in 2004.
But the files show his cooperation with the CIA and MI6 may have been more extensive than previously thought, analysts say.
UN envoy arrives in Libya
A special envoy of the United Nations Secretary General has arrived in Libya, to try to boost international efforts in the country's redevelopment.
Ian Martin will hold discussions in Tripoli on how the UN can assist the Libyan National Transitional Council.
He says the future leaders of Libya face a very big challenge, and the international community should help them in whatever way they ask.
