International and regional envoys are heading to South Sudan in an attempt to end violence and instability after a week of fighting between rebels and government troops.
Diplomats from the United States and Nigeria go to Juba this week to encourage talks between opposing factions.
United Nations Secretary-General Ban Ki-moon called on South Sudan leaders Sunday to find a "political means" to address the conflict. He said the continued violence poses a "dangerous threat" to the future of the young country.
Clashes began last week in the capital, Juba, after President Salva Kiir, a member of the Dinka ethnic group, accused former vice president Riek Machar, a Nuer, of attempting a coup. The government says more than 500 people have been killed, and the unrest has forced tens of thousands of people to flee their homes.
Meanwhile, a South Sudan military spokesman says the government has lost control of Bentiu, the capital of oil-rich Unity State. However Juba officials say the rebels are not in control of any oil fields in the state, and the fields are still operating.
Earlier, South Sudan's government said rebels had overrun the town of Bor in Jonglei state - the scene of some of the worst fighting over the past week.
The U.N. Mission in South Sudan said in a statement Sunday that all non-critical staff in Juba are being evacuated to Uganda as a precautionary measure.
On Saturday, the U.S. military said unidentified gunmen shot at U.S. aircraft flying in to evacuate Americans from Bor. Four U.S. service personnel were reported wounded.
About 35,000 civilians are believed to have fled to U.N. compounds since the unrest began.
South Sudan is the world's newest country, gaining independence from Sudan in 2011.