South Sudanese arch-foes President Salva Kiir and rebel chief Riek Machar are set to formally sign a peace deal at a summit of regional leaders in Ethiopia next week, a top mediator said on Saturday.The rivals have already approved a deal that was negotiated during weeks of talks hosted in Khartoum between South Sudan's warring factions.Kiir and Machar are now expected to ink the final accord at a summit in Addis Ababa on September 12, Sudanese Foreign Minister Al-Dierdiry Ahmed told Sudan's official news agency SUNA."After intense discussions between IGAD leaders in Beijing, it has been decided that an IGAD summit will be held on September 12 in Addis Ababa," Ahmed said, referring to the East African bloc that has pushed the latest initiative to end the war in South Sudan. "Among other issues, the summit will approve the South Sudan peace agreement, which has already been initialled in Khartoum."Machar and Kiir have already signed several other agreements, including a permanent ceasefire and a power-sharing deal that sees Machar returning as first vice president.A succession of peace deals have been signed between the two leaders previously, only to be broken thereafter.South Sudan, the world's youngest country, spiralled into a devastating civil war a little more than two years after it became independent from Sudan in 2011.The conflict pitted Kiir against Machar after the president accused his one-time deputy of plotting a coup.The war has killed tens of thousands of people and uprooted roughly a third of the country's population. The violence has included the widespread rape and murder of civilians, often along ethnic lines.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.