Some 45,000 people have taken refuge in and around UN bases in the country and 81,000 people have fled their homes as a result of fighting that has raised fears of a civil war in the world's newest country.
The US, Norway and Ethiopia are leading efforts to open peace talks between president Salva Kiir and the political rivals he accuses of mounting a coup.
The leaders of Kenya and Ethiopia have arrived in Juba for talks with Mr Kiir aimed at brokering a deal.
China, which has extensive energy interests in South Sudan, also announced it was sending an envoy to the country.
Violence began spreading across South Sudan after a fight among presidential guards on December 15, pitting soldiers from Mr Kiir's Dinka ethnic group against those from the Nuer ethnic group of former vice-president Riek Machar.
Clashes have spread to half of South Sudan's 10 states and the UN is investigating reports of mass killings.