The leader of South Sudan's rebels, former vice president Riek Machar, has said he is not completely in charge of his forces, which have been accused of atrocities during a brutal six-month conflict.

In an interview with the AFP news agency in the Kenyan capital Nairobi on Saturday, Machar was asked if he was in control of his troops and replied: "No, I can't say that. I would be lying to you if I did say that."

"So I can't say I control them, but we are hoping we can control them because we are training them and we are disciplining them," Machar said.

Machar's rebels have been battling forces loyal to President Salva Kiir since December 15, when fighting between rival army factions broke out in the capital Juba.

Kiir accused Machar of having attempted a coup but has in turn been accused of starting the war by launching a purge of his rivals.

Machar admitted his forces had been cobbled together.

"When did we become an army? We were forced out of Juba, those who rebelled ... It took us time to regroup them into a viable force under a control and command" (structure), Machar told AFP.

"We also have volunteer fighters - civilians who have their own guns - who joined the war," he said.

The two sides agreed to a ceasefire in January and again earlier this month, but the truces have not held.

The civil war has claimed thousands of lives, with more than 1.3m people forced to flee their homes.

Some 75,000 people are also sheltering inside UN bases in fear of ethnic violence.

Peace talks are scheduled to resume in the Ethiopian capital Addis Ababa on Wednesday, with the East African regional bloc the Inter-Government Authority on Development (IGAD) providing mediation.

Speaking after talks in Nairobi with Kenyan President Uhuru Kenyatta, Machar stressed he was "committed to peace" to end what he called a "senseless war" in the world's youngest country.

The conflict, which started as a personal rivalry between Kiir and Machar, has divided the army and communities along ethnic lines, pitting members of Kiir's Dinka tribe against Machar's Nuer.

Both sides have been accused of massacres and revenge killings of civilians and the UN has called on both leaders to punish those responsible.