The White House on Monday prodded the leaders of Afghanistan to prove to their people and to the watching world that the results of an intensely disputed presidential election are legitimate.

That reaction came as U.N.-backed fraud investigators on Monday threw out nearly a third of President Hamid Karzai's votes from the country's August election, setting the stage for a runoff between him and his top challenger.

It was unclear whether the Afghan-led Independent Election Commission would accept the findings of the fraud panel and announce a runoff. That could mean a delay in forming a new government, hampering the U.S.-led effort to combat a growing Taliban insurgency.

White House press secretary Robert Gibbs said he would not get ahead of the election review process, but he pointedly framed some expectations.

"It's going to be incredibly important for the world to see that Afghan leaders are willing to make this process legitimate, and that's the process that we're encouraging," Gibbs told reporters. "And I think it's now up to the Afghans to make this legitimate."

He declined to comment on the impact if Karzai does not fully cooperate, saying he did not want to get "too far down the hypothetical highway."

President Barack Obama is leading a lengthy review of the war strategy in Afghanistan. The White House has also said no decision on sending more U.S. troops to Afghanistan would be made before the election crisis is resolved _ a stance reiterated by the civilian chief of the NATO military alliance on Monday.

"None of this is going to work without credible partners," Gibbs added. "We certainly understand that in this entire process."

Two international officials familiar with a new investigation by the U.N.-backed Electoral Complaints Commission told The Associated Press that the findings showed Karzai falling below the 50 percent threshold needed to avoid a runoff with his chief rival, former Foreign Minister Abdullah Abdullah.

Preliminary results released last month showed Karzai winning the Aug. 20 election with more than 54 percent. However, allegations of voter coercion and ballot box-stuffing prompted the fraud investigation and held up a final proclamation of a winner.