John Kerry has held talks in Afghanistan in an effort to broker a resolution to a disputed election that threatens to stir up ethnic tensions and undermine a peaceful political transition.
The US secretary of state held talks on Friday with presidential candidate Abdullah Abdullah and was expected to meet Hamid Karzai, the incumbent president, in the evening.
He also plans to meet the other presidential candidate, Ashraf Ghani, and UN officials during the visit.
The Independent Election Commission declared Ghani the winner of the second round of voting on June 14 with 56.44 percent of the vote, a difference of almost a million votes, according to preliminary results.
The tally might change when the final official numbers are released on July 22.
Abdullah, who won the first round of voting, rejected the preliminary results as a "coup" against the Afghan people, saying the result was invalid because it did not throw out all the fraudulent votes.
"The results that were announced on Monday are preliminary; they are neither authoritative nor final, and no one should be stating a victory at this point in time," Kerry said before his meeting with Abdullah.
Kerry has warned that any effort to resolve the dispute through violence or any "extra-constitutional means," would cause the US to withdraw assistance to Afghanistan.
While the US is drawing down its military presence in Afghanistan, it provides billions of dollars in aid, which helps to fund the operations of the Afghan government.
The suspension of that aid could have a significant impact on the Afghan government's ability to function.
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Following Friday's meeting, Abdullah thanked the US for its support for Afghanistan.
"At a very critical time you have proved your commitment to Afghanistan, to saving Afghanistan, and saving the democratic process here," Abdullah said, referring to Kerry.
Jeff Rathke, State Department spokesman, said Kerry, who has urged both presidential contenders to show leadership at such a critical stage, will press for a thorough review "of all reasonable allegations of fraud," which would entail doing significant additional audits.
"While the United States does not support an individual candidate, we do support a credible, transparent and inclusive process that affirms the Afghan people's commitment to democracy, and that produces a president who can bring Afghanistan together and govern effectively," Rathke said.
Abdullah, the son of a Pashtun father and a Tajik mother, is a former anti-Taliban resistance fighter.
For his part, Ghani, a former World Bank official and ex-finance minister, has strong support from Pashtun tribes in the south and east.