United Nations Secretary-General Ban Ki-moon is calling on both rebels in South Sudan and the country's government to agree to an immediate cease-fire.
Speaking at a news conference Friday, the U.N. chief said he had asked President Salva Kiir to advance the negotiations by agreeing to a key rebel demand.
"I called President Salva Kiir yesterday again and urged him to demonstrate leadership and political flexibility by immediately releasing political prisoners. South Sudan is at a crossroads; this crisis can be resolved only at the negotiating table."
Mr. Kiir's government has refused to release 11 political detainees as demanded by the rebels. The issue is believed to be holding up a proposed cessation of hostilities agreement at peace talks in Ethiopia.
The U.N. Security Council Friday also urged Mr. Kiir to release the political prisoners and called on both sides to immediately stop all hostilities.
Fighting between rebel and government forces continues in South Sudan, where on Friday the army seized control of Bentiu, the formerly rebel-held capital of Unity State.
Army spokesman Philip Aguer told VOA that government forces are also marching on Bor, the rebel-held capital of Jonglei state.
Pro-rebel Brigadier General Lul Ruai Koang said the rebels withdrew from Bentiu to avoid civilian casualties. He said rebel forces remain in control of Bor as well as Unity State's oil installations.
Fighting in South Sudan over the past month has killed at least 1,000 people and triggered fears the world's newest country will descend into all out civil war.
The U.N. refugee agency said Friday that more than 230,000 South Sudanese are now internally displaced and another 43,000 have fled to neighboring countries, with the bulk going to Uganda.
The agency said Uganda is now receiving between 4,000 and 5,000 South Sudanese per day.