Libya: Gaddafi regime's US-UK spy links revealed
The papers suggest the CIA abducted several suspected militants from 2002 to 2004 and handed them to Tripoli.
The UK's MI6 also apparently gave the Gaddafi regime details of dissidents.
The documents, found by Human Rights Watch workers, have not been seen by the BBC or independently verified.
Meanwhile, the head of Libya's interim governing body, the National Transitional Council, said its soldiers were laying siege to towns still held by Col Gaddafi's forces.
Mustafa Abdel Jalil said Sirte, Bani Walid, Jufra and Sabha were being given humanitarian aid, but had one week to surrender.
The BBC's Jon Leyne in Benghazi says there have been unconfirmed reports that Bani Walid has now been taken by anti-Gaddafi forces.
But witnesses on the edge of Bani Walid say the opposition fighters are still on the outskirts although our correspondent adds that it appears as if Gaddafi loyalists have abandoned many of their outlying positions.
'Protecting Americans'
Thousands of pieces of correspondence from US and UK officials were uncovered by reporters and activists in an office apparently used by Moussa Koussa, who served for years as Col Gaddafi's spy chief before becoming foreign minister.
He defected in the early part of the rebellion, flying to the UK and then on to Qatar.
Rights groups have long accused him of involvement in atrocities, and had called on the UK to arrest him at the time.
The BBC's Kevin Connolly in Tripoli says the documents illuminate a short period when the Libyan intelligence agency was a trusted and valued ally of both MI6 and the CIA, with the tone of exchanges between agents breezy and bordering on the chummy.
Human Rights Watch accused the CIA of condoning torture.
"It wasn't just abducting suspected Islamic militants and handing them over to the Libyan intelligence. The CIA also sent the questions they wanted Libyan intelligence to ask and, from the files, it's very clear they were present in some of the interrogations themselves," said Peter Bouckaert of HRW.
The papers outline the rendition of several suspects, including one that Human Rights Watch has identified as Abdel Hakim Belhaj, known in the documents as Abdullah al-Sadiq, who is now the military commander of the anti-Gaddafi forces in Tripoli. Continued…