All nine were US soldiers, a defense official said. “They were all Americans,” the US official said.

The cause was not immediately clear.  The Taleban claimed to have shot down the helicopter, but NATO said there were no reports of hostile fire.

It happened in Zabul province — rugged terrain where helicopters are heavily used to transport military troops spread over mountainous areas with few roads.

One other coalition service member, an Afghan National Army soldier and a US civilian were injured.

So far this year, 525 US and NATO forces have been killed in Afghanistan, surpassing the 504 killed last year.

This year has been the deadliest for international forces since the war began in 2001.

The helicopter crash occurred in northwestern Zabul province in the early morning hours, according to a NATO official.

Mohammad Jan Rasoolyar, a spokesman for the provincial governor in Zabul, said the helicopter went down in Daychopan district.

The NATO contingent in Zabul is dominated by US and Romanian forces.

“I was sitting taking my tea,” said Nakeemullah, 20, who works transporting livestock in the area. “I heard noise and I went outside to see what happened.

“I saw a lot of smoke in the sky,” said Nakeemullah, who uses only one name. “It was far away for me, but I could see that it was a helicopter and it went down on the backside of the mountain where I couldn’t see.” NATO said there were no reports of enemy fire in the area.

However, Taleban spokesman Qari Yousef Ahmadi told the Associated Press that insurgents shot down the helicopter.

Rasoolyar said NATO told local officials that the helicopter crashed due to mechanical problems.

“The Taleban were not involved in this crash at all,” he said.

Also Tuesday, NATO said Afghan and NATO forces had conducted an operation Sunday and Monday to disrupt the Taleban’s freedom of movement outside its heartland of Kandahar city, also in the south, killing at least 11 insurgents and destroying several improvised explosive devices.

Most helicopter crashes in the country have been accidents caused by maintenance problems or factors such as dust.

Also Tuesday, a suspected US missile strike killed seven alleged militants in eastern Afghanistan’s Paktika province, three Pakistani intelligence officials said. The province lies across the border from Pakistan’s South Waziristan tribal region, a major militant stronghold that the Pakistani Army has tried to clear.

NATO said it had no operational reporting on an incident of that description.

Two of the three Pakistani intelligence officials said the militants killed Tuesday belonged to an insurgent group led by Maulvi Nazir. Nazir is believed to have an agreement with Pakistani authorities that they will leave him alone so long as his men avoid attacks on Pakistani soil.