The U.S. military said four American troops were killed and two injured in a collision of two helicopters in southern Afghanistan Monday morning.
Hostile fire was ruled out in the crash. No further details were released.
In an unrelated incident, another helicopter went down Monday during a joint, international security force operation against insurgents in western Afghanistan in which a dozen militants were killed.
The U.S. said military casualties were reported and a recovery operation was under way.
Afghan president, challenger endorse runoff
On Sunday, President Hamid Karzai and his challenger ruled out a power-sharing deal before Afghanistan's Nov. 7 runoff, saying the second round of balloting must be held as planned to bolster democracy in this war-ravaged country.
Some Obama administration officials had said the U.S. would be receptive to a deal to avoid another disruptive election if Karzai and former Foreign Minister Abdullah Abdullah agreed.
However, both Afghan candidates said on talk shows televised Sunday in the United States that they were committed to a second-round vote, despite the huge security and logistical challenges and the threat of Taliban attacks against voters.
"It has to be held. I made sure to have agreement from all the international players before agreeing to runoff to have a second round absolutely surely agreed upon and promised," Karzai said on CNN's "Fareed Zakaria_GPS" program. "Therefore, we must have a second round. If we don't do that, we'll be insulting democracy and a pledge to respecting the vote of the people."
Speaking on "Fox News Sunday," Abdullah was asked if he were interested in a deal to avoid a runoff.
"No, I think I should rule it out because I'm ready to go for a runoff," he replied.
Hoping for legitimate government
President Barack Obama's administration is hoping the runoff will produce a legitimate government after massive ballot-rigging sullied the first-round vote. Another flawed election would cast doubt on the wisdom of sending tens of thousands more U.S. troops to support a weak government tainted by fraud.
Last Tuesday, Karzai bowed to intense international pressure and agreed to a runoff after a U.N.-backed panel voided enough of his votes in the Aug. 20 election that he fell below the 50 percent threshold for a first-round victory in the 36-candidate race.
Abdullah said his focus was on making sure that the November election is carried out without the fraud that marred the first round of balloting.
Abdullah's campaign aides have called for the top three officials on Afghanistan's Independent Election Commission to step down because of their alleged role in electoral fraud. However, Abdullah's campaign has stopped short of threatening a boycott if the electoral officials refuse to step down.
Karzai acknowledged fraud had taken place but insisted that the August balloting "as a whole was clean, and as a result was clear.
"I decided for peace, for stability, and for the future of democracy in Afghanistan and for the future of institutional order in Afghanistan to call for a runoff, and I find that in the interest of the Afghan people," he said.
Karzai said that if Abdullah is interested in joining the government after the election "he is most welcome."
"I'm known for consensus building and for inclusivity," Karzai said. "And that's a good trademark."
‘Same deteriorating situation’
Abdullah, speaking on CNN's "State of the Union," program, said he left the Karzai government three years ago because he did not want to be part of "the same deteriorating situation."
The political battle is raging as the Taliban is gaining strength, exploiting public disenchantment with poor governance and widespread discontent in much of the country over the presence of foreign military forces. Western officials are eager to see a new government in place so they can begin trying to heal the deep rifts within anti-Taliban groups in the country.
In the capital of Kabul, riot police fired shots in the air Sunday to disperse a demonstration by nearly 1,000 students protesting the rumored desecration of a Muslim holy book, the Quran, by U.S. troops during an operation two weeks ago in Wardak province.
U.S. and Afghan authorities have denied any such desecration and insist that the Taliban are spreading the rumor to stir up public anger. The rumor has sparked similar protests in Wardak and Khost provinces.
Students burned an effigy of Obama and chanted slogans such as "down with Americans, down with Israel" as they marched from Kabul University to the parliament building, where riot police turned them back. There were no reports of casualties.
One of the demonstrators, who gave his name as Zabiullah, said "the people behind this humiliation should be arrested."
"The actions of the university students are because the Quran is the law of our Islamic religion, and it has been desecrated," he said.
Also Sunday, the NATO command said a bomb killed an American service member the day before in southern Afghanistan. No further details were released.
The latest death brought to at least 34 the number of U.S. service members killed in Afghanistan this month — down from the record monthly highs this summer but still well ahead of the monthly averages of last year.