Helicopter crashes killed 14 Americans on Monday in the deadliest day for the U.S. mission in Afghanistan in more than four years.

In the first crash, a helicopter went down in the west of the country after leaving the scene of a firefight with insurgents, killing 10 Americans — seven troops and three Drug Enforcement Administration agents, said U.S. officials. The casualties mark the first DEA deaths in Afghanistan since the drug agency began operations there in 2005.

Eleven American troops, one U.S. civilian and 14 Afghans were also injured in the crash.

In a separate incident, two U.S. marine helicopters collided in flight before sunrise over the southern province of Helmand, killing four American troops and wounding two more, marine spokesman Capt. Bill Pelletier said.

It was the heaviest single-day loss of life since June 28, 2005, when 16 U.S. troops on a special forces helicopter died when their MH-47 Chinook helicopter was shot down by insurgents.

Hostile fire ruled out for collision

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.

U.S. military spokeswoman Elizabeth Mathias said coalition forces had launched an operation to recover the wreckage of the helicopter that was downed in the west.

She said the aircraft was leaving the site of a joint operation with Afghan forces when it went down.

The joint force had "searched a suspected compound believed to harbour insurgents conducting activities related to narcotics trafficking in western Afghanistan," NATO said in a statement.

"During the operation, insurgent forces engaged the joint force and more than a dozen enemy fighters were killed in the ensuing firefight."

Afghanistan is the world's largest producer of opium and the illicit drug trade is a major source of funding for Taliban and other insurgent groups.

Deadly year

U.S. forces also reported the death of two other American troops 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 46 the number of U.S. troops 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 Obama administration is debating whether to send tens of thousands more troops to the country, while the Afghan government is rushing to hold a Nov. 7 run-off election between President Hamid Karzai and challenger Abdullah Abdullah after it was determined that the August election depended on fraudulent votes.

The Obama administration is hoping the run-off 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.