Libya's interim rulers on Sunday promised a fair trial for Seif al-Islam, son of Muammar Gaddafi, as his captors vowed not to hand him over to Tripoli until a central government is formed.

World powers are urging Libya to work with the International Criminal Court (ICC), which wants to try the 39-year-old for crimes against humanity allegedly committed in crushing anti-regime protests.

After three months on the run, Seif al-Islam was caught in Libya's far-flung Saharan south early on Saturday in a trap set by a Zintan brigade of militiamen loyal to the new regime.

The National Transitional Council (NTC), struggling to establish its judiciary, is under mounting pressure to ensure a fair trial for him after images emerged of his father dying following his capture on October 20.

"At the moment, he is being held in Zintan. We are going to guarantee the treatment of prisoners under international law," said Osama Juili, head of the Zintan military council.

Britain, France and the United States all called on Libya's new rulers to cooperate fully with the ICC.

And Russia's special representative for Africa, Mikhail Margelov, clearly articulated the concerns of the international community.

"We are happy that this time the new authorities in Libya did not resort to summary justice for Seif al-Islam, the son of the ousted leader Gaddafi," he said after Seif was flown to Zintan following his capture.

Juili said the decision whether to keep Seif al-Islam in detention in the hill town of Zintan or to transfer him to another place would be taken by the new government, although he did not elaborate.

Under its roadmap for the transition, the NTC announced on October 24 that it expects to appoint a transitional government within a month, followed eight months later by the election of a transitional assembly.

Meanwhile, more details about Seif al-Islam's capture emerged on Sunday, with one member of the Zintan brigade expressing surprise at the courage shown by Muammar Gaddafi's longtime heir apparent.

When they were ambushed, Seif and the five people with whom he was travelling in a two-car convoy "did not realise at first what they were dealing with."

"They were afraid at first to be shot, but we must acknowledge that Seif al-Islam surprised us with his calmness and courage.

"We surprised them. They didn't have the time to resist" being captured, he said, adding Seif and his men were armed with little more than "Kalashnikovs, light automatic rifles and grenades."

Video footage showed Seif being hauled off into captivity in Zintan after getting off a flight from the desert, but despite the presence of an angry mob, he was spared the brutal lynching dealt out to his father.

Libya's interim prime minister Abdel Rahim al-Kib moved quickly to give the assurance that Seif would be brought fairly to justice.

"I reassure our people and the world that Seif, and those with him, will be given a fair trial in which international rights and norms will be guaranteed," Kib told a news conference in Zintan.

"The judicial authorities will communicate with the International Criminal Court to examine where Seif al-Islam must be tried," he said.

"Any cooperation with international institutions is welcome," he added. implying that they would like him to be tried in Libya.

Kib said Seif's capture marked a "historic" day and the start of a new chapter for the north African nation.

"The arrest of Seif al-Gaddafi crowns the sacrifices made by our great people," he said in reference to the uprising launched in mid-February that ended Gaddafi's 42-year rule.

"We are going to show Seif al-Islam who we are -- we are not some armed band, we are freedom-seekers," he said.

The head of the Zintan brigade which arrested Seif said that Gaddafi's favourite son asked his captors "to fire a bullet to his head and take him (dead) to Zintan."

"Seif al-Islam was arrested at 1:30 am (2330 GMT Friday)" in the Wadi al-Ajal district, near Awbari in Libya's south, Al-Ajmi al-Atiri told a news conference.

"We received a tip-off from one of Seif's bodyguards. He told us he was thinking of fleeing to Niger," Atiri said.

"We set a trap and laid in wait. Two cars turned up with six people in them, including Seif. We arrested them with little resistance.

News of Seif's capture was greeted with celebratory gunfire in Libya's major cities.

A Libyan television channel, Al-Ahrar, broadcast footage of Seif heavily bearded and leaning on the end of a bed, with three fingers of his right hand bandaged and a blanket on his legs.

Atiri said the injuries came in a NATO air strike.

The urbane Gaddafi son, who was normally clean-shaven in his days at the heart of the regime, was long seen as a potential reformer from within but proved a diehard apologist of brutal repression in the face of the Arab Spring uprising.

The ICC issued warrants on June 27 against Seif al-Islam, as well as his father and Abdullah al-Senussi, the late dictator's intelligence chief, on charges of crimes against humanity in crushing anti-regime protests.

A court spokesman said Libya had an obligation to surrender Seif but did not exclude the possibility of a trial in Libya.