A South Sudan army general has been killed in an ambush as government forces advance on the rebel-held town of Bor.
In Ethiopia, meanwhile, efforts to get the warring parties in South Sudan to agree a ceasefire continue.
Detailed talks between negotiating teams are due to start on Sunday.
The conflict pits army units loyal to President Salva Kiir against rebel forces under sacked Deputy President Riek Machar.
Fighting began on 15 December after the president accused Mr Machar of attempting a coup against him – which he denies.
At least 1,000 people have been killed and nearly 200,000 displaced in the conflict, which has taken on ethnic overtones. Mr Kiir is from the Dinka community and Mr Machar from the Nuer tribe.
Up until Friday, the talks in the Ethiopian capital, Addis Ababa, were conducted by mediators. Now, teams representing the opposing factions are expected to negotiate face-to-face.
A preliminary meeting was held late on Saturday.
Key issues are establishing a ceasefire, and the rebels’ demand for the release of what they see as political prisoners.
Fighting is continuing near Bor, to the north of the capital, Juba, and in Bentiu near the border with Sudan.