Afghanistan's two presidential candidates have agreed to a historic deal to audit all eight million votes cast in the disputed election after two days of intense shuttle diplomacy by top US diplomat John Kerry.

An audible gasp rippled through a packed news conference held on Saturday at the UN headquarters in Kabul as Kerry made the surprise announcement after hours of waiting, saying that vote-checking would begin within the next 24 hours.

Kerry announced that all ballots cast during the June 14 presidential runoff will be audited as the two runners, Ashraf Ghani Ahmadzai and Abdullah Abdullah, had reached an agreement to audit all votes and, regardless of the victor, to form a unity government.

"First, with respect to the election, both candidates have committed to participate in and to abide by the results of the largest, most comprehensive audit. Every single ballot that was cast will be audited 100 percent," Kerry said.

"This is the strongest possible signal by both candidates to restore the legitimacy of the process and the Afghan democracy," he said.

Kerry said the candidates, as well as President Hamid Karzai, are committed to compromise. "They have supported the constitutional process," he said.

The audit will be conducted in accordance with the highest international standards, Kerry said at the briefing, which was also attended by the UN secretary-general's special envoy for Afghanistan Jan Kubis.

"Second, the audit will be carried out in Kabul and will begin in 24 hours. It will start with the ballot boxes currently located here, and ballot boxes in provinces will be transported to Kabul by ISAF (International Security Assistance Forces) and secured by ISAF and Afghan national security forces," Kerry said.

Ghani, who lagged well behind Abdullah in the first-round vote in April, urged Afghans to be patient.

"We will abide by the will of the people. We will not defend any single fraudulent vote," he said.

Abdullah, wearing a suit in contrast to Ghani's traditional Afghan dress, said the two sides had reached a "technical and political agreement".

"I hope this is for the benefit of the Afghan people," he said.

Afghanistan's third presidential election since 2001 was held on April 5. Eight politicians contested the race, with two front-runners - former foreign minister Abdullah Abdullah and former finance minister Ashraf Ghani Ahmadzai - remaining in the runoff.

The preliminary results of the June 14 runoff announced by the election commission on July 7 showed Ghani garnered 56.44 percent of more than 8 million votes, while Abdullah secured 43.56 percent.

The controversial vote results created a deadlock after Abdullah, who bagged 45 percent of 7 million votes cast in the first round on April 5, to Ghani's 31.6 percent, accused the election commission of siding with Ghani. Any decision of the election body is unacceptable, Abdullah said, unless the clean votes are filtered from the fake ones.

Kerry also said that since the audit process would take a couple of weeks, "we request to postpone the inauguration process" of the new government that was initially set for Aug 2.

The audit will be conducted by the UN mission in the country, Kerry said.

AFP - Xinhua

US Secretary of State John Kerry (left) and Afghan presidential candidates Abdullah Abdullah (center) and Ashraf Ghani Ahmadzai (right) attend a news conference announcing an audit of the votes cast in the country's general election at the United Nations Compound in Kabul on Saturday. Jim Bourg / Agence France-Presse