The leaders of two of South Sudan's neighbours, Kenya and Ethiopia, have arrived in the capital, Juba, in an attempt to halt fighting there.
Thousands of people are feared dead in violence that began 11 days ago and has raised fears of a civil war.
President Salva Kiir was seen going into talks with Ethiopian PM Hailemariam Desalegn and Kenyan President Uhuru Kenyatta on Thursday.
Mr Kiir is involved in a power struggle with ex-deputy Riek Machar.
The fighting has exposed ethnic divisions in the world's youngest state as the president is an ethnic Dinka, while Mr Machar represents the Nuer tribe.
UN officials believe thousands have been killed since violence erupted on 15 December, when clashes broke out at a meeting of the ruling Sudan People's Liberation Movement (SPLM).
President Kiir accused the former vice-president, who had been sacked in July, of plotting a coup. Mr Machar denies trying to seize power.
Clashes have since spread to half of South Sudan's 10 states.