Documents found in the abandoned Tripoli office of Muammar Gaddafi's former foreign minister and intelligence chief indicate that US and British spy agencies helped his regime persecute Libyan dissidents, Human Rights Watch said.
The documents were uncovered by the human rights activist group in abandoned offices once occupied by Moussa Koussa, one of Gaddafi's closest associates who left Libya for the UK in February as the uprising against the now-toppled Libyan leader began.
The group said on Saturday it had uncovered hundreds of letters between the US's Central Intelligence Agency (CIA), the UK's MI6 secret service and Koussa, who is now believed to be in Qatar.
Letters from the CIA began, "Dear Moussa", and were signed informally with first names only by CIA officials, Human Rights Watch said.
According to other documents, the UK also invited two of Muammar Gaddafi's sons to the headquarters of its Special Air Service (SAS) special forces unit as Tony Blair, the then-prime minister, tried to build ties with the Libyan regime, The Sunday Times reported on Sunday.
The paper said one secret document showed that Robin Searby, Blair's defence co-ordinator on Libya, had sent a confidential invitation in 2006 for Khamis and Saadi Gaddafi to watch "VIP demonstrations" of the SAS and its sister regiment, the Special Boat Service (SBS).
"There can be no publicity at all connected with this visit, either here or in Libya," it quoted Searby as writing in the letter, found in Saadi Gaddafi's abandoned office in the Libyan capital.
Britain's Ministry of Defence said the visits did not go ahead.
"The article alleges that they were invited on two particular dates in 2006. We have checked and no such visits took place," a spokesperson told the AFP news agency.
Documents uncovered by Human Rights Watch also indicated that Abdel Hakim Belhadj, the current military commander for Tripoli of Libya's National Transitional Government (NTC), was captured and sent to Libya by the CIA.
"Among the files we discovered at Moussa Koussa's office is a fax from the CIA dated 2004 in which the CIA informs the Libyan government that they are in a position to capture and render Belhadj," Peter Bouckaert, from Human Rights Watch, who was part of the group that found the stash, told Reuters news agency.
"That operation actually took place. He was captured by the CIA in Asia and put on a secret flight back to Libya where he was interrogated and tortured by the Libyan security services."
The files shed new light on the practise known as rendition, used by the US under President George W Bush, in which terrorism suspects were handed over to other countries for interrogation.
Rights groups have criticised the US for sending suspects to countries where they were likely to be tortured.
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.
Western intelligence services began co-operating with Libya after Gaddafi abandoned his programme to build unconventional weapons in 2004. But the files show his co-operation with the CIA and MI6 may have been more extensive than previously thought, analysts say.
'Protection from terrorism'
The depth of the ties could anger officials in Libya's provisional government, many of whom are long-term opponents of Gaddafi and are now responsible for charting a new path for Libya's foreign relations.
"Our concern is that when these people were handed over to the Libyan security they were tortured and the CIA knew what would happen when they sent people like Abdel Hakim into the hands of the Libyan security services," Bouckaert said.
In Washington, CIA spokeswoman Jennifer Youngblood, without commenting on any specific allegation or document, said: "It can't come as a surprise that the Central Intelligence Agency works with foreign governments to help protect our country from terrorism and other deadly threats. That is exactly what we are expected to do."
A US official, speaking on condition of anonymity, added: "There are lots of countries willing to take terrorists off the street who want to kill Americans. That doesn't mean US concerns about human rights are ignored in the process."
A British government spokesperson told Reuters that Britain did "not comment on intelligence matters."
More recent documents showed that after the war broke out six months ago, Libya reached out to a former rebel group in the breakaway Somali state of Puntland, the Somali Salvation Front, asking them to send 10,000 fighters to Tripoli to help defend Gaddafi.