Members of South Sudan's government and rebels have begun peace talks in the Ethiopian capital of Addis Ababa.
The meetings are aimed at stopping the fighting that has left more than 1,000 people dead and displaced more than 190,000.
The two groups have yet to meet face-to-face, but have separately met mediators from the East African bloc IGAD.
The government and rebels have agreed in principle to a ceasefire, but neither side has indicated when they might lay down their arms.
A rebel spokesman said its forces were marching on towards the capital Juba after taking control of the city of Bor in the state of Jonglei.
His comments came a day after the Sudan People's Liberation Army, which is loyal to the government, said its forces were advancing on Bor.
Meanwhile, the US Embassy in Juba has ordered the evacuation of more staff from the country.
In an emergency message to US citizens, the embassy said the move was due to the deteriorating security situation.
It said there would be an evacuation flight today arranged by the Department of State.
"We are not suspending our operations. We are just minimising our presence," said ambassador Susan Page.