South Sudan's warring factions have agreed to reunify their political party, a conflict resolution organization said Thursday.

South Sudan President Salva Kiir and rebel leader Riek Machar signed the agreement to unite the ruling Sudan People's Liberation Movement Wednesday in the Tanzanian town of Arusha, the Conflict Management Initiative said.

"This is not a final peace agreement but it is an agreement meant to address the root causes of the conflict which started in the SPLM," said Akol Paul Kordit, chairman of the SPLM Youth League. "This agreement is a watershed in the history of the people of South Sudan. It is a cornerstone in our search for peace."

Others who also signed the agreement were Deng Alor Kuol, a representative of former detainees and Tanzanian President Jakaya Kikwete, whose government helped mediate the talks.

Fighting broke out December 2013 between Kiir's troops and those loyal to Machar, who is Kiir's former vice president. Since then, more than 1.5 million South Sudanese have fled their homes. The two sides have signed several peace deals brokered by neighboring governments, but none has actually stopped the warfare in the oil-rich country.

Much of the violence has pitted the Dinkas, who back Kiir, against the Nuer, who support Machar. Gross atrocities were committed by both sides, including the killings of elderly patients in hospital wards as well as the slaughter of hundreds of civilians in the capital last December, according to human rights groups.

Amnesty International and three other organizations said in a statement Thursday that to help bring accountability, the African Union should immediately publish the report of the AU Commission of Inquiry which looks into atrocities committed in the South Sudan conflict.

"Three months after the Commission of Inquiry submitted its report to the AU, its findings and recommendations are yet to see the light of day," said Edmund Yakani, director of Community Empowerment for Progress Organization.

South Sudan peacefully broke away from Sudan in 2011 after decades of war, becoming the world's newest nation.

___

Odula reported from Nairobi, Kenya.