In the deadliest crash, a helicopter went down in the west of the country after leaving the scene of a firefight, killing 10 Americans - seven troops and the three DEA agents. Eleven American troops, one U.S. civilian and 14 Afghans were also injured.
In a separate incident, two U.S. Marine helicopters - one UH-1 and an AH-1 Cobra - collided in flight before sunrise over the southern province of Helmand, killing four American troops and wounding two more, Marine spokesman Maj. Bill Pelletier said.
It was the heaviest single-day loss of life since June 28, 2005, when 19 U.S. troops died, including 16 on an MH-47 Chinook helicopter that was shot down by insurgents.
U.S. authorities have ruled out hostile fire in the collision but have not given a cause for the other fatal crash in the west. Taliban spokesman Qari Yusuf Ahmedi claimed Taliban fighters shot down a helicopter in northwest Badghis province's Darabam district. It was impossible to verify the claim and unclear if he was referring to the same incident.
Military spokeswoman Elizabeth Mathias said hostile fire was unlikely because the troops were not receiving fire when the helicopter took off.
NATO said the helicopter was returning from a joint operation that targeted insurgents involved in "narcotics trafficking in western Afghanistan."
"During the operation, insurgent forces engaged the joint force and more than a dozen enemy fighters were killed in the ensuing firefight," a NATO statement said.
U.S. forces also reported the death of two other American service members a day earlier: one in a bomb attack in the east, and another who died of wounds sustained in an insurgent attack in the same region. The deaths bring to at least 47 the number of U.S. service members who have been killed in October.
This has been the deadliest year for international and U.S. forces since the 2001 invasion to oust the Taliban. Fighting spiked around the presidential vote in August, and 51 U.S. soldiers died that month - the deadliest for American forces in the eight-year war.
The latest deaths came as President Barack Obama prepared to meet his national security team for a sixth full-scale conference on the future of the troubled war.
Obama is debating whether to send tens of thousands more troops to the country, while the Afghan government is rushing to hold a Nov. 7 runoff election between President Hamid Karzai and challenger Abdullah Abdullah after it was determined that the August election depended on fraudulent votes.
The administration is hoping the runoff will produce a legitimate government. In Washington, Obama was to meet with his national security team Monday in what was to be the sixth full-scale Afghanistan conference in the White House Situation Room.
Abdullah on Monday called for election commission chairman Azizullah Lodin to be replaced within five days, saying he has "no credibility."
Lodin has denied accusations he is biased in favor of Karzai, and the election commission's spokesman has already said Lodin cannot be replaced by either side.
Abdullah made the demand in a news conference during which he spelled out what he said were "minimum conditions" for holding a fair second round of voting, including the firing of any workers implicated in fraud and the suspension of several ministers he said had campaigned for Karzai in the first round before the official campaigning period began.
Abdullah did not say what would happen if his demands were not met. "I reserve my reaction if we are faced with that unfortunate situation," he said.
Abdullah said he was willing to meet with Karzai to discuss the conditions, but repeated that he would not discuss a coalition government as some have suggested, nor compromise on his recommendations out of concerns that they are difficult to implement.
"These are not impossible things," Abdullah said, stressing that his team had pared them down to what they considered essential to a fair vote and possible to put in place before the runoff.
Another flawed election would cast doubt on the wisdom of sending in more U.S. troops.
With less than two weeks to go until the vote, disagreements have emerged between the U.N. and the Afghans on how to conduct the balloting.
Lodin said the commission hopes to open all 23,960 polling stations from the first round. The U.N. wants to open only 16,000 stations to cut down on the number of "ghost polling stations" that never opened but were used to stuff ballot boxes.
Elsewhere Monday, Nangarhar province Gov. Gul Agha Sherzai survived an assassination attempt after a gunman fired automatic weapons at his convoy in Jalalabad, according to his spokesman Ahmad Zia Abdulzai. Sherzai's bodyguards killed the gunman, as well as another attacker wearing a suicide vest and carrying grenades.
Meanwhile, security forces in Kabul fired automatic rifles into the air for a second day Monday to contain hundreds of stone-throwing university students angered over the alleged desecration of Islam's holy book, the Quran, by U.S. troops during an operation two weeks ago in Wardak province. Fire trucks were also brought in to push back protesters with water cannons. Police said several officers were injured in the mayhem.
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.
Associated Press Writers Rahim Faiez, Todd Pitman and Robert H. Reid contributed to this report from Kabul; Noor Khan reported from Kandahar and Devlin Barrett from Washington.