Dozens of American soldiers have died in Afghanistan this year
At least 14 Americans have been killed in a series of air crashes in Afghanistan, military officials say.
Four US soldiers died and two were hurt when two helicopters collided mid-air in the south, Nato-led forces said.
In a separate helicopter crash, in western Badghis province, seven US soldiers and three US civilians died.
The US said hostile fire was not believed to be to blame for either incident, but the Taliban said it was behind the Badghis crash.
Taliban spokesman Qari Yusuf Ahmedi told the AFP news agency that militants had shot down the US helicopter in the Darabam district of the province.
Another 12 Americans and 14 Afghans were also reported to have been injured in that crash.
Col Wayne Shanks, a spokesman for the Nato-led force in Afghanistan, said the authorities were still investigating the incident in Badghis, but "do not believe that enemy action was responsible".
He said the crash happened as troops were leaving an area where they had carried out an anti-drugs trafficking operation and killed 14 "enemy fighters".
"After they were flying away, that's when the crash actually happened," he said. "We did go... and remove an insurgent who was working with the drug trade in that particular area."
The three dead civilians worked for the US Drug Enforcement Administration, news agencies reported.
The deaths were the first for the DEA in Afghanistan since it began operations there in 2005, Associated Press reported.
Bad year for troops
The exact location of the crash in the south of the country has not been confirmed by the US military.
This year has seen the highest death toll of international troops in Afghanistan since the Taliban were overthrown in 2001.
There have been dozens of American soldiers among those killed, making up more than half the total foreign troop deaths in the country.
The worst single loss of US life in Afghanistan occurred in June 2005, when a Chinook helicopter was shot down in eastern Kunar province, killing 16 military personnel.