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.
The International Committee of the Red Cross said it expects to visit Seif in Libya soon.
"We think we will be able to visit him soon," Yves Daccord, the ICRC director general, told journalists in Geneva. "We are in contact with National Transitional Council and have made a request to get access to Seif al-Islam."
He called Kadhafi's son "someone who must be protected," adding that the ICRC had requested access not only to Seif but also to all the other detainees in Libya.
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.
After three months on the run, Seif was caught early on Saturday in southern Libya in a trap SET by a Zintan brigade of militiamen loyal to the new regime.
Ex-spy Senussi was captured the next day, also in the south, NTC 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.
The ICC issued warrants on June 27 against Seif, 39, Kadhafi and Senussi on charges of crimes against humanity allegedly committed when security 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, the ousted leader's brother-in-law, 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.