KABUL, Afghanistan -- Afghanistan's rival presidential candidates reached a breakthrough agreement Saturday to complete an audit of their disputed election and a national unity government.

The deal, brokered by U.S. Secretary of State John Kerry, offers a path out of what has threatened to be a debilitating political crisis for Afghanistan, with both candidates claiming victory and talking of setting up competing governments.

Such a scenario could have dangerously split the fragile country's government and security forces at a time the U.S. is pulling out most of its troops and the Taliban continue to wage a fierce insurgency.

Instead, former Finance Minister Ashraf Ghani Ahmadzai and former Foreign Minister Abdullah Abdullah agreed to abide by a 100 percent, internationally supervised audit of all 8 million ballots in the presidential election. They vowed to form a national unity government once the results are announced, presumably one that includes members of each side.

Kerry, who conducted shuttle diplomacy between the two candidates late into the night Friday and Saturday, warned that much work still remained.

"This will be still a difficult road because there are important obligations required and difficult decisions to be made," Kerry said after briefing Afghanistan's current president, Hamid Karzai.

The audit, which comes after widespread fraud allegations, is expected to take several weeks, beginning with the ballot boxes in the capital, Kabul.

Boxes from the provinces will be flown to the capital by helicopter by U.S. and international forces and examined on a rolling basis. Representatives from each campaign as well as international observers will oversee the review, and the candidate with the most votes will be declared the winner and become president.

Both candidates agreed to respect the result, and the winner would immediately form a national unity government. The inauguration, which had been scheduled for Aug. 2, would be postponed, with Karzai staying on a little longer as president.

The two candidates spent Saturday inside the U.S. Embassy building, holding separate meetings with Kerry, according to campaign officials. Talks continued into the early evening without food or drink because of Ramadan, during which Muslims fast during the day.

Discussions concerned how to determine how many ballots should be re-examined for fraud and how to ensure that the audit was impartial. Both sides had wrangled over the technical details, in particular the threshold for selecting which ballots should be reviewed.

Abdullah said at a news conference announcing the deal that the election created "serious challenges." But he praised Ahmadzai for working toward the accord for the audit and unity government.

Ahmadzai returned the compliments, lauding his competitor's patriotism and commitment to a dialogue that promotes national unity.

"Stability is the desire of everyone," he said. "Our aim is simple: We've committed to the most thorough audit" in history. Such a process would remove any ambiguity about the result, he added.

When the two candidates were done speaking, they shook hands and hugged. Kerry later joined them as they raised their arms together in triumph.

"This is unquestionably a tense and difficult moment," Kerry said, "but I am very pleased that the two candidates who stand here with me today and President Karzai have stepped up and shown a significant commitment to compromise."

The announcement came as a relief to a country worried about how the election dispute would resolve itself. Both the full audit and the agreement to form a unity government drew praise from television commentators immediately after the speeches.

The prolonged uncertainty about the outcome of the election had jeopardized a central plank of President Barack Obama's strategy to leave behind a stable state after the withdrawal of most U.S. troops at year's end.

Preliminary runoff results, released last week against U.S. wishes, suggested a turnaround in favor of Ahmadzai, a former World Bank economist. He had lagged significantly behind Abdullah in the first round of voting.

In the first round of voting on April 5, Abdullah emerged the winner against 11 other candidates, with 45 percent of the vote to Ahmadzai's 31 percent. But because neither won more than 50 percent of the vote, a required runoff between the two was held June 14. Preliminary results from the runoff show Ahmadzai leaping ahead with 56 percent of the vote, and Abdullah with 44 percent.

Abdullah, a top leader of the Northern Alliance that battled the Taliban before the U.S.-led invasion in 2001, claimed widespread ballot-stuffing. He was runner-up to Karzai in a fraud-riddled 2009 presidential vote before he pulled out of that runoff, and many of his supporters see him being cheated for a second time. Some, powerful warlords included, have spoken of establishing a "parallel government."

Ahmadzai's team has said that fraud took place on both sides and insisted that his better showing in the runoff was the result of an energetic campaign to mobilize his fellow ethnic Pashtuns to vote for him.

Kerry and Karzai discussed the deal past midnight Saturday. When they emerged early this morning, the Afghan leader endorsed the outcome.

Speaking alongside Karzai at the Presidential Palace, Kerry said the democracy springing up in Afghanistan "deserved its full bloom." He offered robust U.S. support to ensure the deal holds.

The United Nations chief in Afghanistan, Jan Kubis, who will direct much of the technical aspects of the audit, delivered his strongest praise for Kerry. He said Kerry's work wasn't typical diplomacy, but almost a "miracle."

Kubis called on other nations to send extra observers to assist with the audit as soon as possible.

Kerry repeatedly stressed in his mediation that Washington isn't taking sides.

Kubis and other officials said the talks in Kabul focused on the technical particulars of the U.N. audit. Kerry spent significant time hammering home the point that each side must come together at the end of the contest for the good of the country.

Extended instability would have immediate consequences for Afghanistan. If no process had been established and both Ahmadzai and Abdullah attempted to seize power, the government and security forces could have split along ethnic and regional lines. The winner in such chaos could be the Taliban, whose battle against the government persists despite the U.S. spending hundreds of billions of dollars and losing more than 2,000 lives since invading the country after the Sept. 11, 2001, terrorist attacks.

The Taliban have intensified their spring offensive in a bid to undermine the Western-backed government. Saturday's breakthrough came after two roadside bombs killed at least 10 people, authorities said.

The Taliban were blamed for the larger attack in Kandahar province, in which a bomb struck a civilian car, killing eight people, Panjwayi district police chief Sultan Mohammad said.

In eastern Nangarhar province, another bomb struck a vehicle in Jalalabad, killing a civilian and a police officer, said Ahmad Zia Abdulzai, a spokesman for the provincial governor. No group immediately claimed responsibility for that blast.

With Iraq wracked by an extremist Sunni rebellion, the Obama administration moved quickly to ensure Afghanistan's political instability also didn't break out into violence. A prolonged crisis also could have had security implications for Washington.

Ahmadzai and Abdullah have vowed to seal a bilateral security pact with the U.S. that Karzai has refused to sign.

The U.S. has said it needs legal guarantees to leave behind about 10,000 troops in Afghanistan next year. If the pact isn't finalized, U.S. officials say they may have to pull out all American forces, an undesired scenario that played out three years ago in Iraq.

Information for this article was contributed by Bradley Klapper, John Daniszewski and staff members of The Associated Press; and by Carlotta Gall and Matthew Rosenberg of The New York Times.

A Section on 07/13/2014