Addis Ababa - South Sudan's warring leaders have been given an August 17 deadline to agree another peace deal to end 19-months of civil war, mediators said on Thursday.Peace talks, led by the eight-nation East African IGAD bloc, have been taking place in Ethiopia almost as long as the war, resulting in at least seven failed agreements and ceasefires, all broken within days or even hours."We hope that the parties have travelled a long way and seen the consequences," top mediator Seyoum Mesfin said, adding that a summit had been scheduled to begin August 5 to agree the deal. Previous meetings have been repeatedly delayed.The August 17 deadline to sign a deal is the latest in a string of ultimatums issued since the civil war started in December 2013, when President Salva Kiir accused Riek Machar, who had been sacked as vice president, of attempting a coup.The conflict was immediately ethnic, pitting Kiir's Dinka people against Machar's Nuer, and quickly spread. It has been characterised by ethnic massacres, rape and the use of child soldiers. Tens of thousands have been killed, according to the UN.The latest peace attempt broke down in March, with both the rebels and government accused of seeking a military solution.A new proposal to set up an "inclusive transitional government" was adopted on Thursday by mediators, even though many of the points on power sharing were previously rejected by both sides in past talks."This is a comprehensive and final agreement that addresses all the problems of South Sudan that led to this crisis: the governance, the management of the economy, the security sector and the power sharing," Mesfin said.UN aid chief Stephen O'Brien warned on Thursday while visiting South Sudan of the "devastating" toll on civilians in a civil war marked by atrocities. Nearly 70% of the country's population - 7.9 million out of 11.6 million people - are expected to face food insecurity in coming months, according to the UN.After the collapse of the last round of talks, mediators expanded involvement to create an "IGAD-plus" group.It now also includes the United Nations, five more African Union nations from across the continent - South Africa, Algeria, Chad, Nigeria and Rwanda - as well as China and the "Troika", Britain, Norway and the United States.