South Sudan rebel chief Riek Machar on Tuesday refused to sign a final peace deal with the government, in a setback for a regional drive to end nearly five years of brutal civil war.South Sudanese arch-foes President Salva Kiir and Machar have held weeks of talks in Khartoum in search of a comprehensive peace deal to end the conflict that has killed tens of thousands of people and displaced millions in the world's youngest country since it erupted in December 2013.The warring parties have already inked several agreements, including a permanent ceasefire and a power-sharing deal that sees Machar returning as first vice president in the government, but on Tuesday the rebel leader refused to sign the final peace deal.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook"The main South Sudanese opposition groups, including the SPLM-IO (Machar faction), refused to sign the final document demanding that their reservations be guaranteed in it," Sudanese Foreign Minister Al-Dierdiry Ahmed told reporters.Officials said the rebel groups had differences over the functioning of a proposed transitional government, how many states the country should be divided into and on the writing of a new constitution."For the first time, the opposition told us that it will not sign," Ahmed said, showing the draft text to reporters and diplomats who had gathered for what was expected to be a preliminary signing ceremony in the Sudanese capital.The rebel groups' refusal to ink the document has come as a setback to the latest peace push led by regional East Africa bloc IGAD."This is the final document which had been arrived at after consultations between all South Sudanese parties," Ahmed said."South Sudan will not have peace unless these groups sign."Challenges aheadThe Sudanese minister said the opposition's refusal to sign spelt the end of the current Khartoum round of talks."This was the last round of negotiation," Ahmed said, adding that the mediators will submit the text to IGAD, although it was unclear when the bloc's leaders would meet to discuss it.Earlier this month, Kiir and Machar signed a power-sharing deal that will see the rebel leader return to the government as the first of five vice presidents. That accord was to pave the way for a final peace deal and the formation of a transitional government that will hold power until elections are held.But international backers of the peace process had raised doubts about whether the deal would stick given the depth of the animosity between South Sudan's leaders which dates back to the 1990s when Machar first broke ranks at the height of its war for independence from Khartoum."Considerable challenges lie ahead, and we are concerned that the arrangements agreed to date are not realistic or sustainable," Britain, France and the US said in a joint statement on August 10. "Given their past leadership failures, South Sudanese leaders will need to behave differently and demonstrate commitment to peace and good governance," they said.South Sudan finally became independent from Sudan in 2011, but a little over two years later a fresh war erupted pitting Kiir against his erstwhile deputy Machar.The conflict has seen widespread rape and murder of civilians, often on ethnic lines, and uprooted roughly a third of the population.A succession of peace deals have been signed between the two leaders only to be broken, most recently in December. Each time, the two sides have traded blame for the breakdown and the resulting bloodshed.