Saadi Gaddafi had been let into Niger on humanitarian grounds and was due in its capital Niamey later, said Niger's Justice Minister Marou Amadou.
Col Gaddafi's whereabouts are unknown. He has said he will die in Libya.
Anti-Gaddafi troops now control most of Libya, including the capital Tripoli. Loyalists are holding out in several cities including Bani Walid and Sirte.
On Sunday, anti-Gaddafi forces resumed their attack on Bani Walid, 180km (110 miles) from Tripoli, supported by Nato air strikes. Officials say their forces are now within reach of the centre of the town.
They also moved closer to the Mediterranean city of Sirte, Col Gaddafi's birthplace.
Guinnea Bissau offer
Some of Col Gaddafi's family have fled to Algeria.
Several convoys of former Gaddafi loyalists are also said to have streamed over Libya's southern border with Niger recently.
The Niger government has recognised the NTC's authority, but said it had not yet decided whether it would allow Col Gaddafi to enter the country.
The US has urged Niger to detain any individuals who may be sought for prosecution by the new authorities in Tripoli and confiscate their weapons and money.
Meanwhile, the poor West African nation of Guinea Bissau has said it would welcome Col Gaddafi if he sought refuge there.
"With all the investment that Gaddafi has put into Guinea Bissau he deserves the respect and good treatment by the authorities and people of Guinea Bissau," said Prime Minister Carlos Gomes Junior over the weekend.
At the beginning of September, Saadi Gaddafi reportedly made contact with the interim authorities, offering to negotiate an end to fighting in Libya. Nothing came of the offer.
Saadi Gaddafi is a former footballer who had a very brief career in Italy, and he ran the Libyan Football Federation after being national team captain. Since retiring from football, he has become involved in the film industry, apparently investing $100m (£60m) in a film fund.
The chairman of the NTC has moved to Tripoli from the eastern city of Benghazi as the interim authorities try to establish their presence.
And Mahmoud Jibril, the head of the NTC cabinet which serves as interim prime minister, has announced that an interim government will be formed within 10 days. All areas of Tripoli would be represented, including those where fighting was continuing, he told reporters on Sunday.
He also announced that Libya had resumed producing oil, saying more would be on stream in the near future.
Also on Sunday, the spokesman for the military council in Tripoli, Anes Sharif, said Colonel Gaddafi's head of foreign intelligence, Abuzeid Dorda, had been arrested in the city.
A long-time associate of the former Libya leader, Mr Dorda served as prime minister in the 1990s, was Libyan ambassador at the UN until 2003 and took over from Moussa Koussa as head of intelligence in 2009.