Uganda has dropped its deadline for the conclusion of peace talks, while rebels continue to gather in assembly points, as agreed in a ceasefire.
Vincent Otti led the first group of rebels from the bush
The rebel Lord's Resistance Army's leader Joseph Kony, is said to be near an assembly point, but it is unclear if he has crossed into southern Sudan.
The rebels have been trekking on foot through northern Uganda since a cessation of hostilities last month.
President Yoweri Museveni had given the LRA until 12 September to reach a deal.
"There's not really a deadline any more," government delegation spokesman Capt Paddy Ankunda told Reuters news agency.
The chief mediator at the talks, Riek Machar, says several hundred rebels and Mr Otti are now at the assembly points - one of which is near Uganda and the other near the Democratic Republic of Congo.
"They have shown up in the western assembly point in Ri-Kwangba. They have also shown up in Owiny Ki-Bul," Mr Machar said from south Sudan capital, Juba.
"Vincent is definitely there, behind the lines Joseph [Kony] himself is there. It is big progress that they have accepted to go to the assembly areas," Mr Machar said.
Uganda's International Affairs Minister Henry Okello told journalists in the capital, Kampala, that Mr Otti had not yet reached the assembly point.
However, he said the government was not concerned, as he had shown that he was on his way out of the bush.
"We will see whether Kony and Otti and the LRA are there, then decide our next move," Capt Ankunda said.
Earlier a senior United Nations official, who attended the peace talks, said the LRA planned to release the women, children and other non-combatants they are holding.
UN Humanitarian Affairs chief Jan Egeland said the UN would help those released by the rebels return to their communities.
The LRA insists war crimes charges should be dropped against its leaders.
According to the terms of the deal, the rebels will be protected by the southern Sudanese and the government has undertaken not to attack them.
The LRA has abducted thousands of children and forced them to fight since the conflict began.
Aid workers say the flood of children - known as night commuters - that used to stream into northern Ugandan towns every night for protection from rebel attacks has now slowed to a trickle.