KABUL: Secretary of State John Kerry sought Friday to broker a deal between Afghanistan's rival presidential candidates as a bitter dispute over last month's runoff election risked spiraling out of control.

Kerry, who arrived predawn in Kabul on a hastily arranged visit, is meeting with former Finance Minister Ashraf Ghani Ahmadzai and former Foreign Minister Abdullah Abdullah as well as the current leader, President Hamid Karzai.

The objective is to convince both candidates to hold off on declaring victory or trying to set up a government until the United Nations can conduct an audit of extensive fraud allegations in the voting.

For the United States, the political crisis is threatening to undermine more than a decade of efforts to leave behind a strong Afghanistan capable of containing the Taliban insurgency and preventing extremist groups like al-Qaida from using the territory to endanger the American homeland.

“We are in a very, very critical moment for Afghanistan,” Kerry told reporters after meeting the UN chief in Afghanistan, Jan Kubis. “Legitimacy hangs in the balance. The future potential of the transition hangs in the balance. So we've a lot of work to do.“

The hope, Kerry said, was to create a process that confers legitimacy on whoever emerges as the rightful leader of Afghanistan. “But I can't tell you that's an automatic at this point.“ For his part, Kubis said the UN was searching for an election result “that strengthens stability and unity in the country, and not the other way around“.

With Iraq wracked by insurgency, Afghanistan's election is posing a new challenge to President Barack Obama's effort to leave behind two secure states while ending America's long wars.

Both Ghani and Abdullah have vowed to sign a bilateral security pact with Washington, which says it needs the legal guarantees in order to leave behind some 10,000 boots on the ground in Afghanistan after most American troops are withdrawn at the end of the year.

If no clear leader emerges, the US may have to pull out all its forces, an unwanted scenario that played out in Iraq just three years ago. Karzai has refused to sign the agreement, leaving it in the hands of his successor. The preliminary runoff results suggested a massive turnaround in favor of the onetime World Bank economist Ghani, who lagged significantly behind Abdullah in first-round voting.

“Our commitment is to ensure that the election process has the integrity and the legitimacy of Afghanistan and the world,” Ghani told reporters as he met with Kerry. Ghani voiced support for the fullest audit possible.

Abdullah, a top leader of the Northern Alliance that battled the Taliban before the US-led invasion in 2001, claims massive ballot-stuffing and his supporters have spoken of establishing a “parallel government”, raising the specter of the Afghan state collapsing.

Abdullah was runner-up to Karzai in a fraud-riddled 2009 presidential vote before he pulled out of that runoff. The winner amid all the chaos could be the Taliban, whose fight against the government persists despite the United States spending hundreds of billions of dollars and losing more than 2,000 lives since invading the country after the Sept. 11, 2001, terrorist attacks.

Many Afghans fear Taliban forces will only gain strength as the US military presence recedes. And internal instability could fuel the insurgency.

Kerry will seek a commitment from both candidates to hold off from rash action while the ballots are examined and political leaders are consulted across Afghanistan's ethnic spectrum. The US wants to ensure that any new government welcomes all ethnic factions.

A UN audit, however rudimentary, probably could be done within two weeks, US officials believe. The focus would be on clear fraud indicators, including districts with high turnout or more women going to the voting polls than men.

James Dobbins, the State Department's special representative for Afghanistan and Pakistan, said some fraud had been expected. But the believed scope of the fraudulent voting appears “quite extensive“.

Speaking in Washington this week, Dobbins said Abdullah's campaign particularly mistrusts the impartiality of the Afghan electoral institutions. Chief electoral officer Zia ul-Haq Amarkhail resigned this week, though he denied wrongdoing.

He cited national interest in resigning. Obama spoke to each candidate this week, warning that any move outside the law to seize power would mean the end of US financial aid to Afghanistan. Before becoming president, Obama differentiated Afghanistan from Iraq, which he declared a “dumb war”.

In contrast, he described Afghanistan a fight worth waging, ordering tens of thousands of new troops into the country in his first year in office. The risk of a prolonged Afghan political crisis has alarmed US officials already struggling to respond to sectarian tensions in Iraq that have broken out in open warfare.

The situations in Afghanistan and Iraq are distinct. But in each, the US has spent years trying to ensure the survival of democratic governments able to effectively police their own territories. In both countries that objective is in peril, their futures threatened by a combination of poor leadership, weak institutions, inter-ethnic rivalry and fierce extremist rebellions.