Afghan presidential candidate Abdullah Abdullah threatened on Tuesday to withdraw from an audit of votes cast in the election, a move that could wreck U.N. efforts to rescue the country's first democratic transfer of power.

The June 14 election has triggered a standoff between Abdullah and his poll rival Ashraf Ghani, with both candidates claiming victory amid allegations of massive fraud.

U.S. President Barack Obama urged Afghanistan's two presidential candidates to finalize negotiations on forming a national unity government as soon as possible, the White House said Sunday.

National Security Council spokeswoman Caitlin Hayden said Obama spoke separately on Friday with Ashraf Ghani and Abddullah Abdullah, rivals to replace Hamid Karzai as president of Afghanistan.

Afghan President Hamid Karzai said Saturday his successor would take office on September 2, despite a slow-moving vote audit designed to eliminate fraudulent ballots cast for the two contenders.

About 200 Taliban fighters launched an offensive in a province near Kabul on Tuesday as Afghanistan's disputed election threatens to leave the central government weakened at the same time as U.S. troops pull out.

President Hamid Karzai appealed for the two men vying to succeed him to end their stand-off over the poll result and save the country from worsening violence.

Afghanistan's feuding presidential candidates signed a deal Friday to form a national unity government, opening an apparent way forward in a dispute over the fraud-tainted election that threatens to revive ethnic conflict.

Ashraf Ghani and Abdullah Abdullah vowed to work together whoever becomes president after an ongoing audit of all eight million votes finally declares the winner of the June 14 election.

A suicide attacker killed an influential cousin of Afghan President Hamid Karzai on Tuesday, officials said, raising tensions during a dispute over election results that will determine the country's new leader as U.S.-led troops withdraw.

Hashmat Karzai was a campaign manager in the southern province of Kandahar for Ashraf Ghani, one of the two presidential candidates involved in a bitter stand-off that threatens to trigger worsening ethnic instability.

U.S. Secretary of State John Kerry Saturday held a second day of talks with Afghanistan's feuding presidential hopefuls, seeking a deal to "clean up the tally" after disputed elections.

Despite back-to-back meetings on Friday with rivals Abdullah Abdullah and Ashraf Ghani and other officials that stretched deep into the night, U.S. officials said an accord was not yet on the table.

Presidential candidate Abdullah Abdullah claimed victory on Tuesday in Afghanistan's disputed election, blaming fraud for putting him behind in preliminary results as fears rise of instability and ethnic unrest.

Abdullah told a rally of thousands of rowdy supporters in Kabul he would fight on to win the presidency, but he called for patience from loyalists who demanded he declare a "parallel government" to rule the country.

Former World Bank economist Ashraf Ghani won Afghanistan's presidential election, according to preliminary results Monday, but fraud allegations are set to spark a dispute over the outcome and stoke concerns of instability.

Ghani won 56.4 percent of the run-off vote to ex-foreign minister Abdullah Abdullah's 43.5 percent -- a major comeback for Ghani after the first-round election in April when eight men stood to succeed President Hamid Karzai.

U.S. Secretary of State John Kerry called Afghan President Hamid Karzai on Tuesday and urged him to address concerns of the rivals vying to succeed him, amid fears an election dispute could trigger instability.

Kerry also "stressed the importance of national unity and a peaceful political process," a State Department spokesperson said, after the preliminary result of Afghanistan's presidential election run-off was delayed for days for a recount of ballots in 2,000 voting centers.