The ceasefire between South Sudan's government and rebels will end months of violence and see the formation of a transitional government.
President Salva Kiir's government and the rebels backing Riek Machar have signed a ceasefire deal, a "critical first step" in peace negotiations.
South Sudan's rebels and a government delegation have begun peace talks in Ethiopia to try end fighting that has killed at least 1 000 people.
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.
After receiving evidence that al-Shabab is planning attacks in the country, Ethiopia has put its security forces on heightened alert, say officials.
Ethiopian Prime Minister Meles Zenawi has died from a sudden infection while recovering from an undisclosed illness at a hospital in Belgium.
The countries still need to resolve key border security issues to end hostilities, a mediator from the African Union said on Saturday.
Nkosazana Dlamini-Zuma has won a tightly contested vote to become the new head of the African Union Commission - the first woman to hold the post.
Sudan and South Sudan signed a security agreement which aims to defuse tensions over oil payments which officials had warned could spark a war.