A top South Sudan rebel general said Tuesday that he and other powerful commanders had split from their chief Riek Machar, rejecting ongoing peace talks and risking a worsening of the country's civil war.

Already more than two dozen armed groups are involved in a 19-month-long civil war that has left tens of thousands dead and has been marked by widespread atrocities on both sides.

Rebel commander Gathoth Gatkuoth, sacked last month along with another key commander, Peter Gadet, said they were now at war with both former rebel comrades and the government in Juba.

Gatkuoth, the former rebel logistics chief, said he and Gadet would now battle Machar and President Salva Kiir.

"They are symbols of hate, division and failed leadership," he said. "Both leaders were responsible for starting the crisis."

South Sudan's civil war began in December 2013 when Kiir accused his former deputy Machar of planning a coup, setting off a cycle of retaliatory killings that has split the poverty-stricken, landlocked country along ethnic lines.

Peace talks in neighbouring Ethiopia restarted last week as international pressure mounts ahead of an August 17 deadline to strike a deal.

But the breakaway rebels accused Machar of seeking power for himself, and said they would not recognise any deal agreed.

"Those talks are about Riek Machar looking for positions, it is not the peace that we want," Gatkuoth told AFP, claiming to be speaking from border regions between South Sudan and Sudan. "Any peace that is signed will not be legitimate and will not be respected."

It is unclear how many troops the new rebel faction command, but both Gatkuoth and Gadet have long been powerful commanders on the ground. It will also likely weaken Machar's position at the talks.

The war has been characterised by ethnic massacres and rape. Recent attacks have included castration, rape and tying children together before slitting their throats.

Gadet -- blacklisted with United Nations sanctions -- operated as a notorious militia chief for decades in the northern battleground state of Unity.

He was accused of shooting down a UN helicopter in August 2014 in which three Russian crew members were killed, claims he denied. He also led an April 2014 attack on the oil town of Bentiu in which hundreds were slaughtered, according to the UN.