The first face-to-face meeting between South Sudan’s President Salva Kiir and rebel leader Riek Machar since mass violence began in December is due to take place in Ethiopia.
The US says it is not optimistic that Friday’s one-day talks will produce an immediate result.
The conflict 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.
South Sudan broke away from Sudan in 2011 after decades of conflict with the Khartoum government.
A peace deal was signed by the two leaders in January but failed to bring an end to the violence.
“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.
Peace mediators in Ethiopia confirmed that Mr Machar arrived in Ethiopia on Thursday in preparation for the talks in Addis Ababa.
Norwegian Foreign Minister Borge Brende said in an interview that the talks between Mr Kiir and Mr Machar should include an outline for an inclusive transitional government.
“Otherwise, there will be consequences that will follow,” he said. “We will increase our pressure on the parties. Even tougher measures will follow in the coming weeks if there is no political will to solve the crisis.”