South Sudan's government says its forces have recaptured Bor, a strategic town that has changed hands several times since fighting between government and anti-government forces erupted in mid-December.
Army spokesman Philip Aguer says SPLA forces entered Bor on Saturday and "defeated" more than 15,000 opposition fighters. He said the military has "frustrated" what he said were plans by former vice president Riek Machar to attack the capital, Juba.
There was no immediate word from opposition fighters on the status of Bor, the capital of Jonglei state.
African regional bloc, the Intergovernmental Authority on Development (IGAD) has been trying to broker a cease-fire between government and anti-government representatives who have been meeting in Ethiopia.
On Saturday, anti-government negotiator Mabior de Garang said the opposition had agreed on a plan and would be willing to sign a cease-fire in the coming days.
The French news agency AFP said the South Sudanese government negotiator was preparing to return to Addis Ababa to sign a cessation of hostilities.
The two sides have been holding talks in Ethiopia for several weeks. The talks had bogged down over an opposition demand for the release of 11 people who were arrested shortly after last month's alleged coup.
It was not immediately clear how the issue had been resolved.
On Friday, U.N. Secretary-General Ban Ki-moon said South Sudan's crisis has reached "tragic proportions."
The unrest has left at least 1,000 people dead and nearly 470,000 displaced.
A December gun battle at the army headquarters in Juba touched off the crisis. President Salva Kiir accused his former deputy Machar of attempting a coup, a charge Machar has denied.