The expected talks, taking place after intense lobbying from world leaders with Washington slapping sanctions on senior military commanders, come a day after the UN warned crimes against humanity had likely be carried out in the still raging conflict.
President Salva Kiir arrived Friday in the Ethiopian capital, where he is slated to meet his former vice president turned arch rival, rebel chief Riek Machar.
Foreign Minister Barnaba Marial Benjamin told reporters he hoped the meeting "would bring peace" and "allow full implementation of the ceasefire agreement", which has been in tatters ever since it was signed in January.
This handout picture taken and released on April 29, 2014 by the United Nations Mission in South Sudan (UNMISS) shows United Nations Special Advisor for the Prevention of Genocide Adama Dieng (L) speaking, along UN High Commissioner for Human Rights, with rebel backed former South Sudanese vice president Riek Machar (R) in an undisclosed location in South Sudan. AFP PHOTO
But a spokesman for Machar, who had swapped his military fatigues for a business suit when he arrived in Addis Ababa late Thursday from his rebel base in South Sudan, said the two leaders would be unlikely to meet immediately.
Both will first hold talks first with host and top mediator, Ethiopian Prime Minister Hailemariam Desalegn.
"I don't think Riek Machar and Salva Kiir will meet directly today," Machar's spokesman James Gadet Dak told AFP.
While both leaders speak of peace, fierce fighting still rages and the United Nations has warned of the risk of severe famine and genocide.
The rebel's military spokesman Lul Ruai Koang claimed heavy clashes Friday in three states, including the key oil-producing states of Unity and Upper Nile.
Killed like chickens
The conflict, which started as a personal rivalry between Kiir and Machar, has seen the army divide along ethnic lines, pitting members of Kiir's Dinka tribe against Machar's Nuer.
The United States this week unveiled its first sanctions in response to the "unthinkable violence", targeting one military leader from each side.
The war has claimed thousands -- and possibly tens of thousands -- of lives, with over 1.2 million people forced to flee their homes.
The UN report detailed horrific killings, including in the first days after fighting broke out in the capital Juba on December 15.
One Nuer man recounted to UN rights workers how army troops raided houses and shot civilians in the city.
"Nuer were being killed like chickens," he was quoted as saying.
"Witness after witness recounted horror as they watched security forces enter their communities, sometimes in tanks and with heavy weaponry, and round up their relatives and neighbours," the report added.
"In some cases, victims were killed immediately; in others, they were taken to other locations and killed."
In other areas, Dinka people were targeted for their ethnicity and killed, including in massacres in the northern oil town of Bentiu, where fighting continues.
Aid agencies are warning that South Sudan is now on the brink of Africa's worst famine since the 1980s, while both US Secretary of State John Kerry and the UN human rights chiefs have spoken of their fears that the country could slide towards genocide.
But as pressure builds to stem the brutal conflict, fears are growing that political leaders can no longer hold back their warring forces as communities spiral into cycles of revenge attacks, Amnesty International said in a report Thursday.
Testimonies in Amnesty's report describe civilians including children executed by the side of the road "like sheep", gang rapes of women using sticks, and other victims "grotesquely mutilated" with their lips sliced off.
In one case, a woman who was three months pregnant was gang-raped by 14 men and then forced to watch as seven women who resisted being raped were killed as gunmen instead forced sticks into their vaginas.
"The longer ethnic rivalries are allowed to deepen and fester, the more fragmented South Sudan will become, making reconciliation and sustainable peace much more difficult to achieve," Amnesty warned.
The conflict erupted on December 15 with Kiir accusing Machar of attempting a coup. Machar then fled to the bush to launch a rebellion, insisting that the president had attempted to carry out a bloody purge of his rivals.