KABUL (AFP) - Afghanistan's two feuding presidential candidates on Friday signed a deal confirming they would form a national unity government after an audit of votes is completed to decide the winner of the fraud-tainted election.

US Secretary of State John Kerry, visiting Kabul to try to mediate an end to the country's election impasse, welcomed the deal as a major advance in bringing Afghanistan back from the brink of political chaos.

The threat of spiralling ethnic unrest has loomed large since Abdullah Abdullah refused to accept a preliminary vote count that put Ashraf Ghani ahead, saying that the June 15 election had been stolen by massive ballot-box stuffing.

"Today myself, our team, and Dr Ghani and his team have taken another step forward in the interests of strengthening national unity in the country... and also to bring hope for the better future for the people of Afghanistan," Abdullah said.

But the two candidates have clashed repeatedly in recent weeks over both the audit and the draft power-sharing plan, and attention will focus on whether all their supporters accept the deal.

"We trust each other," Ghani said. "We will work with each other to fulfil this national duty and obligation for every Afghan.

"We are affirming that we will form a government of national unity... What unites us is far greater than what divided us during the campaign.

"He and I have reached an agreement and signed a communique for future cooperation."