In a day of military tragedy and political drama, 11 American troops and three U.S. civilians died Monday in two helicopter crashes in...

KABUL, Afghanistan — In a day of military tragedy and political drama, 11 American troops and three U.S. civilians died Monday in two helicopter crashes in rural Afghanistan, while President Hamid Karzai and his top political rival escalated their dispute over conditions for holding a runoff election set for Nov. 7.

Presidential candidate Abdullah Abdullah demanded that the nation’s top election official and three Cabinet ministers be fired before the runoff, but Karzai refused. The disagreement threatens to derail an election that is crucial to American military strategy in Afghanistan.

As the nation waited tensely for the electoral contest, the two helicopter crashes marked one of the deadliest days for U.S. forces since combat operations against the Taliban and al-Qaida began here eight years ago.

U.S. military officials here said one helicopter crashed in western Afghanistan after it took off from the site of an anti-drug raid and a firefight with Taliban insurgents. The crash killed seven U.S. troops and three agents of the U.S. Drug Enforcement Administration. In the second incident, in southern Afghanistan, two NATO helicopters collided in flight, killing four American troops.

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The officials said no enemy attack was involved in either incident but that both were under investigation. A spokesman for the Taliban said the insurgents had shot down a helicopter in the western province of Badghis, but his claim could not be confirmed. The exact locations of the two incidents were not released.

“These separate tragedies today underscore the risks our forces and our partners face every day,” said Col. Wayne Shanks, a spokesman for the U.S.-led NATO coalition. “Our grief is compounded when we have such a significant loss on one day.”

Meanwhile, in a reminder of deepening public antagonism to the Western troop presence here, police battled rock-throwing, anti-American college protesters in Kabul for a second day. The students denounced the reported destruction of a Quran by U.S. forces. American military officials here have denied the reports and blamed them on Taliban propaganda.

Preparations continued Monday for the runoff vote between Karzai and Abdullah, which is being held because the original election Aug. 20 was discredited by revelations of massive fraud, costing Karzai nearly a million votes and leaving neither of the top two candidates with enough votes to win.

United Nations officials here said they hoped Afghan officials would do everything possible to hold an election “cleaner” and fairer than the first, and they asked that both candidates stick to their promises to accept a runoff after the fraud findings left the first round inconclusive.

But the escalating standoff between the two candidates left open the possibility that Abdullah might withdraw from the race, rendering it irrelevant and potentially throwing into doubt the legitimacy of the government, just as U.S. officials are trying to decide whether to commit tens of thousands more troops to the fight against the Taliban and al-Qaida.

At a news conference here, Abdullah called for the removal of the head of Afghanistan’s Independent Election Commission, the suspension of three Cabinet ministers, the removal of certain provincial police chiefs and the closure of about 500 polling stations.

He said he would give Karzai five days to comply with the demands, which he called the “minimum conditions for having transparent and credible” elections. But he remained coy about whether he would quit the race if Karzai did not meet his demands.

Just a few hours later, Karzai issued a brief statement saying he would not remove either the election-panel chairman or the Cabinet ministers before the election.

Abdullah charged that the officials were biased toward Karzai or had helped orchestrate the fraud. He also asked for the closure of hundreds of “ghost” polling stations, where security problems prevented monitors from overseeing the vote.

Even some of Abdullah’s aides said his demands were probably unrealistic. Some Western officials saw his move as a tactic to bolster his position, whether he intends to participate in the runoff or seek a negotiated coalition government with Karzai.