U.S. soldiers from the 2nd Battalion, 12th Infantry Regiment, 4th Brigade Combat Team, 4th Infantry Division fire mortars Monday in the Pech Valley of Afghanistan’s Kunar province.

Kabul ? 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 scheduled 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, killing seven U.S. troops and three three agents of the U.S. Drug Enforcement Administration. In the second incident, in southern Afghan-istan, two NATO helicopters collided in flight, killing four American troops.

The officials said that there was no enemy attack 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 Koran by U.S. forces. American military officials here have denied the reports and blamed them on Taliban propaganda.

Preparations continued Monday for the runoff 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 that was “cleaner” and fairer than the first round, and they asked that both Karzai and Abdullah 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.