South Sudan's Kiir and Machar meet in Ethiopia
South Sudan's President Salva Kiir and rebel leader Riek Machar are meeting for the first time since mass violence began in December.
The meeting is being held in Addis Ababa, and both men first held separate talks with Ethiopia's prime minister.
The conflict in the world's newest state has left thousands dead and more than one million homeless.
The UN has accused both sides of crimes against humanity, including mass killings, sexual slavery and gang-rape.
"Widespread and systematic" atrocities were carried out in homes, hospitals, mosques, churches and UN compounds, a UN report said on Thursday, calling for those responsible to be held accountable.
An estimated five million people are in need of aid, the UN says.
A cessation of hostilities deal was signed by both sides in January but failed to bring an end to the violence.
A 30-day truce was supposed to have taken effect on Wednesday.
The US has said it is not optimistic that Friday's one-day talks will produce an immediate result.
Mr Machar arrived on Thursday in preparation for the talks in Addis Ababa, while President Kiir flew in on Friday.
Ethiopian Prime Minister Hailemariam Desalegn agreed to mediate the talks proposed by US Secretary of State John Kerry after his visit to the region last week.
Discussions are expected to centre on ending the fighting and power sharing.
South Sudan ministers have said the government's priority is to stop the violence and discuss a "transitional process".
However, Foreign Minister Barnaba Marial Benjamin was quoted by Reuters news agency as saying that a transitional government would not be discussed, and that Mr Kiir would remain leader until the 2015 elections.
The release of and dropping of treason charges against four top South Sudanese politicians is said to have paved the way for talks.
The men's release had been a key demand of the rebels.
"I don't believe that [the two sides] will reach an agreement straight away," US Ambassador to South Sudan Susan Page said during a radio call-in show.
"But if they can agree on a broad-based process on how to resolve the conflict, end the fighting, that would be a step forward."
Ms Page said that people wanted peace and could not understand why the country should have descended into war barely three years since independence.
Correspondents say far-reaching international sanctions could be imposed against both sides if there is no discernible progress in reaching an agreement.
The violence began when President Kiir accused his sacked deputy Mr Machar, of plotting a coup.
Mr Machar denied the allegation, but then marshalled a rebel army to fight the government.
The battle assumed ethnic overtones, with Mr Machar relying heavily on fighters from his Nuer ethnic group and Mr Kiir from his Dinka community.
The UN has about 8,500 peacekeepers in South Sudan, which became the world's newest state after seceding from Sudan in 2011.
However, they have struggled to contain the conflict, and the government has accused the UN mission of siding with the rebels.
It denies the allegation.
South Sudan gained independence in 2011, breaking way from Sudan after decades of conflict between rebels and the Khartoum government.
It remains one of the world's poorest countries.