Addis Ababa, Ethiopia: South Sudan President Salva Kiir and rebel leader Riek Machar late Sunday signed a new agreement to end more than 13 months of fighting in a civil war that has left tens of thousands dead.

"Complete cessation of hostilities in South Sudan is expected as of this morning (Monday)," Seyoum Mesfin, a negotiator from the regional IGAD bloc, told reporters in the Ethiopian capital Addis Ababa where the ceasefire deal was signed.

The two leaders have signed and then broken at least six previous ceasefire agreements since fighting began in December 2013.

The eight-member East African bloc IGAD, which has in the past threatened sanctions over violations but never taken action, says this time it will take any ceasefire violations to the UN Security Council and the African Union's Peace and Security Council and ask for "tough measures" against them, Mesfin said.

After four days of difficult negotiations in the Ethiopian capital, Kiir and Machar have however failed to reach agreement on the conflict settlement proposed by IGAD which provides for a power-sharing arrangement between the two men.

"This is a partial agreement because we have not solved some of the most critical issues," Machar said after the signing, citing disagreements on the "transitional government structure" to set up and divide responsibilities within the administration.

Negotiations will resume on February 20 with IGAD giving the warring sides one last chance to reach a final agreement by March 5.

Fighting broke out in South Sudan, the world's youngest nation, in December 2013 when Kiir accused his sacked deputy Machar of attempting a coup.