Moamer Kadhafi's son Seif al-Islam and his spymaster Abdullah Senussi were on Monday both behind bars as Libya's new leaders moved to bring the former regime's two most wanted men to justice.

The National Transitional Council, Libya's interim authority, insists that Seif, arrested on Saturday in the country's far-flung Saharan south, be brought to trial in Libya where he could face the death penalty.

NTC officials have yet to indicate their intentions regarding Senussi, who has been held at a secret location and not been seen in public since his capture early on Sunday.

But they are likely to want to see Senussi also tried at home rather than at the International Criminal Court (ICC) in The Hague, where both he and Seif face charges of crimes against humanity.

World powers, fearful that Seif would not be given a fair trial after his father was felled by a bullet to the head after being captured on October 20, are urging Libya to work with the ICC.

Interim prime minister Abdel Rahim al-Kib gave assurances that Seif was being well treated.

"We assure all those concerned that he (Seif) is in good hands. The treatment he is entitled to is very good, a hundred times better than what he and his father meted out to the Libyan people," Kib told a news conference in Tripoli.

The formation of a new government, apparently delayed by Seif's capture, will now be announced on Tuesday, he said. "We are working hard to ensure to have something solid, coherent, capable of doing the job."

UN human rights chief Navi Pillay, meanwhile, urged Libya's interim rulers to ensure that Seif and Senussi are treated humanely, and to cooperate fully with the ICC.

On Sunday, interim justice minister Mohammed al-Allagui told AFP that Kadhafi's son would be tried in Libya "because local justice is the rule and international justice is the exception."

"We have the necessary guarantees for a fair trial, especially after the amendment of a law that guarantees the independence of the judiciary as regards the executive," he said.

ICC spokesman Fadi Al-Abdallah, however, said the Libyan authorities were obliged to cooperate with the court and surrender Seif, although he could still be tried in Libya.

"If the Libyan authorities want to hold the trial in Libya, they must submit a request to the ICC and the judges will decide," he said.

"According to the principle of complementarity and the Statue of Rome, the priority rests with national law."

After three months on the run, Seif was caught early on Saturday in a trap set by a Zintan brigade of militiamen loyal to the new regime.

Ex-spy Senussi was captured in the south on Sunday, officials said.

Bashir Uweidat, who heads the southern Wadi Shati military council, said Senussi "did not put up any resistance" and was arrested by former rebels at his sister's home in the Al-Guira region.

Ghoga confirmed the arrest of Kadhafi's brother-in-law, who is also wanted by the ICC.

The court issued warrants on June 27 against Seif, 39, Kadhafi and Senussi on charges of crimes against humanity allegedly committed when Kadhafi's forces crushed a popular uprising in February.

In particular, it accused Senussi, 62, of being an "indirect perpetrator of crimes against humanity of murder and persecution based on political grounds" committed in Benghazi.

Senussi has been described by the ICC as "one of the most powerful and efficient organs of repression of Moamer Kadhafi's regime."

He is also wanted in France where a Paris court sentenced him in absentia to life in 1999 over an attack on a French UTA airliner a decade earlier that killed 170 people.

World powers have repeatedly urged Libya's new rulers to respect international norms in dealing with prisoners amid reports of abuse, and are now urging the NTC to cooperate fully with the ICC.

Zintan military council chief Osama Juili said Seif would be held there until a new transitional government decides his fate.

"At the moment, he is being held in Zintan. We are going to guarantee the treatment of prisoners under international law," Juili said. It was not immediately clear where Senussi was being held after his capture.