Lord Resistance Army (LRA) number two, Vincent Otti, who with elusive rebel supremo Joseph Kony and other top commanders are wanted on international war crimes charges, arrived at the site in southern Sudan with a band of fighters, said Riek Machar, the lead mediator of stalled peace talks.
Mr Machar, the vice president of autonomous southern Sudan, told reporters that Kony and other senior members of the LRA leadership were close behind, as he welcomed top UN relief envoy Jan Egeland to the peace talks' venue in Juba, Sudan.
"The leadership of the LRA is there," he said, referring to Ri-Kwangba in southern Sudan's Western Equatoria State, where rebels holed up in the neighbouring Democratic Republic of Congo are to assemble under the truce.
"Vincent is definitely there, and behind the lines is Joseph himself,"
Mr Machar said.
Joseph Kony is a self-proclaimed prophet and mystic who has led the shadowy rebel group for 19 years.
The truce, which took effect on August 29 and boosted hopes that the peace talks could end northern Uganda's brutal, two-decade war, gave the LRA three weeks to gather at Ri-Kwangba and another site near the Ugandan border.
The first rank-and-file fighters began arriving at the Owiny-Ki-Bul camp at the weekend and Mr Machar said those there now numbered about 400, but that the LRA leadership had initially stayed away amid complaints of minor truce violations.
Otti's presence in Ri-Kwangba is likely to assuage fears that the truce is in danger and may pave the way for a resumption of negotiations, which have been stalled since the ceasefire came into force.
Under the terms of the truce, the rebels, who number between 500 and 5,000 according to widely varying estimates, are to stay in the camps under the supervision of the south Sudan government for the duration of the talks.
Mr Machar's announcement came a day ahead of a deadline -- now almost certain to be extended -- set by Ugandan President Yoweri Museveni for the rebels to agree to a deal or lose an amnesty offer for the charges they face.
But the talks will not start up for at least another week as the LRA delegation to the talks has asked for a break to consult their leaders on the next phase of negotiations, which are to focus on power-sharing, Mr Machar said.
He spoke as Mr Egeland, who spent the weekend touring war-ravaged northern Uganda, arrived in Juba to press the two sides on ending the conflict that has killed tens of thousands of people and displaced nearly two million others.
The UN undersecretary-general for humanitarian affairs said he had come to Juba to offer help to Mr Machar's mediation efforts and assist with the return from the bush of thousands of rebel wives, children and presumed abductees.
Speaking at a news conference on Monday, Mr Egeland said rebels had agreed to release so-called "non-essentials," women, children, the wounded, elderly and sick into the care of the United Nations.
"We are ready to receive and care for them and help them to return to their communities," he said, adding that he had high hopes for the Juba peace talks.
"I have been convinced this is the best chance ever to end one of the worst wars of our generation," said Mr Egeland, who frequently describes northern Uganda as one of the world's worst and most-forgotten humanitarian crises.
The LRA, which took leadership of a regional rebellion in 1988 and purports to want to create a government based on the Biblical Ten Commandments, has been accused of committing horrendous atrocities in its fight.
Kony, Otti and three other senior rebel commanders, one of whom is now dead, have been charged with war crimes by the International Criminal Court and are demanding the indictments be quashed before signing a peace deal.
President Museveni has said he will not ask the tribunal based in The Hague to consider dropping the charges until an agreement is finalised.