The warring parties last month signed a preliminary powersharing deal which they are expected to finalise in Khartoum on Sunday, that will see Machar return to a unity government as the first of five vice presidents.

"This agreement will not collapse and I am sure that it will not collapse because the people of South Sudan have now agreed that they must make peace among themselves," Kiir told journalists.

He was speaking after meeting Sudanese Foreign Minister Mohamed Aldirdiri who is mediating the peace talks, and confirmed the final agreement would be signed on August 5.

South Sudan's nearly five year conflict began after Kiir accused his sacked vice president Machar of plotting a coup against him in 2013.

Machar returned to government as vice-president in April 2016 after a power sharing deal, but fighting between the rival sides erupted in Juba just three months later with Machar fleeing into exile where he has remained ever since.

The conflict has already killed tens of thousands of people and forced millions from their homes and resisted numerous ceasefires, leaving the country's economy in ruins.

While Kiir said he was "committed" to the deal, he highlighted several challenges going forward, especially in accommodating a bloated government.

The deal stipulates that there will be 35 ministers in the transitional government -- 20 from Kiir's group and nine from Machar's, while the rest will represent other groups.

The parliament will be comprised of 550 lawmakers, including 332 from Kiir's group and 128 from Machar's faction.

"They need security, they need vehicles, they need houses ... five vice presidents, this is a very big responsibility to manage. I need to get for them their transport, and one person needs a motorcade of maybe five vehicles. Where will I get this?" said Kiir.

"They also need houses where will I get houses, also they need offices so that they work, where will I get offices. So there are so many things need to be done."