Rebel leadership wants defector returned and tried for crimes against humanity once Gaddafi is toppled

Libya's rebel leadership has called for Moussa Koussa, the former Libyan foreign minister who has defected to the UK, to be returned for trial for murder and crimes against humanity after Muammar Gaddafi is toppled.

Mustafa Gheriani, a spokesman for the revolutionary council in its de facto capital, Benghazi, said that the rebels were not bent on revenge against the regime's officials but that some of Gaddafi's closest associates "have a lot of blood on their hands" and must stand trial.

The British foreign secretary, William Hague, has said that Britain is not offering Koussa immunity from prosecution, and called for other regime figures to abandon Gaddafi.

Gheriani alleged that Koussa had been partly responsible for assassinating opposition figures in exile, murderous internal repression and the Lockerbie plane bombing.

"We want to bring him to court," Gheriani said. "This guy has so much blood on his hands. There are documented killings, torturing. There's documentation of what Moussa Koussa has done. We want him tried by Libyan people. I believe once we have our government 100% in control in Libya, things are normalised, we want him tried here. I think international law gives us that right."

Gheriani said it was up to Britain to decide whether to arrest Koussa in the meantime. Koussa's arrival in London was evidence that Gaddafi's regime was "starting to crumble". He expected other senior officials to follow.

"He is a very, very major person to defect. Gaddafi trusted him more than some of his sons. Now Gaddafi doesn't even trust his own people any more," Gheriani said.

Gheriani said a senior military official in Kufra, Colonel Saleh al-Zaroug, had defected to the rebels. He had served in an army division commanded by one of Gaddafi's sons. The defection was impossible to confirm.

Hague said Koussa was not being offered any immunity from British or international justice. He had come to the UK on a private plane from Tunisia having left Libya of his own free will. "Gaddafi must be thinking to himself: 'Who will be the next to walk away?'" Hague said.

It would not be "helpful to advertise" whether other senior members of the regime planned to quit, Hague said, but he believed it was likely that many opposed Gaddafi's actions. He added that Koussa was in a secure place in the UK and talking voluntarily to British officials, including staff at Britain's Tripoli embassy now based in London.

Koussa's defection provides Britain with an unparalleled source of intelligence on the state of the Libyan ruler's inner circle. But his arrival in the UK has also led to expectations that he will be questioned about his possible involvement in or knowledge of atrocities including the Lockerbie bombing and the murder of PC Yvonne Fletcher.

Jim Swire, whose daughter, Flora, was killed in the Lockerbie bombing, said Mousa's defection was a "fantastic day" for the victims' families.

"Within the Libyan regime, [Koussa] is in the best position of anyone other than Gaddafi himself to tell us what the regime knows or did," he said. "He would be a peerless source of information."

Koussa was expelled from the UK in 1980 and became the head of Libyan foreign intelligence for 15 years, including the period of the Lockerbie bombing, which happened in 1988. He has always denied Libya was involved.

Jack Straw, a former foreign secretary, has described Koussa as a key player with a "fundamentally important" role in negotiations to bring Libya back into the international fold. "Moussa Koussa's apparent defection, certainly his unscheduled visit here, will be a very important factor in just adding to the weight against the Gaddafi regime and tipping the balance against him," Straw told the BBC. "From a distance, what's clear is that there is unlikely to be any military victory for either side. So it does depend on which side psychologically collapses."

Alongside Koussa's defection, it has emerged that Barack Obama signed a secret government order authorising covert US help to the Libyan rebels.

Opposition fighters are trying to recover some of the territory retaken by a government counterattack that has again brought Gaddafi's forces within striking distance of Benghazi.

The revolutionary leadership has called for more of the air strikes that allowed it to surge towards Gaddafi's home town of Sirte. But after their tanks and artillery were destroyed by the coalition, government troops switched tactics, using armoured pickup trucks to outflank and ambush the ragtag opposition forces.

Rebels trying to recapture the town of Brega came under rocket and mortar fire early on Thursday.