The Intergovernmental Authority on Development announced the deal from Addis Ababa, Ethiopia, where the government and rebels had been negotiating, CNN reported.
The rebels, led by former Vice President Riek Machar, had accused negotiators for South Sudanese President Salva Kiir Mayardit of "intransigence," the Sudan Tribune reported.
James Gatdet Dak, a spokesman for Machar, said the agreement had been delayed because Kiir would not agree to the release of political prisoners and the withdrawal of Ugandan troops as soon the cease-fire went into effect.
The government negotiating team did not appear at the talks Wednesday morning.
Clashes between government forces and rebel troops began Dec. 15 after Machar attempted to stage a coup.