South Sudan's warring parties are holding preliminary meetings before the official start of negotiations in a bid to end nearly three weeks of conflict, Ethiopia's Foreign Ministry has said.
Dina Mufti, a spokesman for the ministry, said on Friday that representatives of the government and rebel groups were meeting in the country's capital, Addis Ababa. He said the meeting was necessary to bridge differences ahead of direct talks expected to start on Saturday.
Meanwhile, fighting intensified on the ground in South Sudan as the army moved on the rebel-held town of Bor, capital of Jonglei state.
"We have enough forces who will defeat the rebels within 24 hours," army spokesman Philip Aguer said amid reports of heavy battles involving tanks and artillery on the outskirts of Bor, which has already exchanged hands three times since fighting began in mid-December.
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"These forces - the rebels - are now retreating back," Aguer said, quashing rebel claims that they had been marching on the capital Juba.
Rebel leader and former vice president Riek Machar told Britain's Telegraph newspaper that his forces would hold back from attacking Juba in the hope of achieving a "negotiated settlement".
Government forces should also stop trying to take territory under his control, Machar added.
The US government said 20 US citizens had been flown out of the country by a military cargo plane. A spokesman for the US embassy in Juba said the evacuation was due to the "deteriorating security situation".
South Sudan's government has declared a state of emergency in Unity and Jonglei, two states whose capitals are under rebel control.
President Salva Kiir has insisted that the fighting was sparked by a coup attempt mounted by soldiers loyal to Machar on December 15.
But that account has been disputed by some officials of the ruling party who say the violence began when presidential guards from Kiir's Dinka ethnic group tried to disarm those from the Nuer group of Machar.
From Juba, violence spread across the country, with forces loyal to Machar defecting and seizing territory from loyalist forces.
South Sudan has been plagued by ethnic tension and a power struggle within the ruling party that escalated after Kiir dismissed Machar as his vice president in July. The rebels back Machar, who is now a fugitive sought by the military.
South Sudan peacefully broke away from Sudan in 2011 following a 2005 peace deal. Before that, the south fought decades of war with Sudan.