Nine members of the Nato-led International Security Assistance Force have died in a helicopter crash in Zabul province, southern Afghanistan.
All nine were US soldiers, it was announced later. A Nato soldier, an Afghan soldier and a US civilian were also injured, Isaf said.
The Taliban said it shot down the aircraft but Isaf said there had been no enemy fire in the area.
At least 529 foreign troops have been killed so far this year in Afghanistan.
With more than three months to go, 2010 is already the deadliest year since the US-led invasion in 2001, according to figures collated by the website iCasualties.
A spokesman for the governor of Zabul told AP news agency the helicopter came down in the province's north-western Daychopan district.
Isaf did not give the exact location of Tuesday's incident.
"The cause of the crash is under investigation," it said. "There are no reports of enemy fire in the area."
An Isaf spokesman told the AFP news agency an operation was underway to recover the aircraft.
The three injured have been taken to an Isaf medical facility for treatment.
There were no reports of insurgent activity in the area, suggesting the helicopter was not shot down, says the BBC's Ian Pannell in Kabul.
Taliban spokesman Zabiullah Mujahid said insurgents had shot down the helicopter, but this could not be substantiated. The Taliban often exaggerates reports of its attacks on foreign troops.
There are almost 150,000 foreign troops fighting the Taliban in Afghanistan, supporting about 300,000 Afghan security forces.