The government and rebels have traded accusations of breaking the ceasefire deal intended to calm violence in South Sudan.
South Sudan's government and rebels have sent delegates to peace talks in Ethiopia but fighting continues as both sides battle for control.
Uganda President Yoweri Museveni says East African nations have agreed to unite if South Sudan rebel leader Riek Machar rejects a ceasefire offer.
The UN Security Council has approved the deployment of troops to almost double the number of peacekeepers in South Sudan.
US aircraft have come under fire while trying to evacuate Americans from the conflict in South Sudan.
African mediators said they held productive talks with South Sudan's President Salva Kiir, to prevent an ethnic civil war.
President Salva Kiir is willing to remove the army from the border with Sudan, signalling a possible compromise with his Sudanese counterpart.