KABUL, Afghanistan – A helicopter carrying international troops crashed in a rugged section of southern Afghanistan on Tuesday, killing nine service members in the deadliest such incident in four years for coalition forces.

A “large number” of Americans were among the dead, according to a senior military official in Washington, who spoke on condition of anonymity. The official was not authorized to discuss the matter publicly.

One other coalition service member, an Afghan National Army soldier and a U.S. civilian were wounded.

The coalition would not disclose the helicopter’s mission, and the cause of the crash was not immediately clear. NATO said there were no reports of enemy fire in the early morning hours in the Daychopan district of Zabul province, where the crash took place.

However, Taliban spokesman Qari Yousef Ahmadi said to The Associated Press by telephone that insurgents shot down the helicopter.

“The Taliban are not involved in this crash at all,” said Mohammad Jan Rasoolyar, a spokesman for the provincial governor in Zabul.

“The investigation is still going on, but the military told us that the helicopter crashed due to technical problems.”

Another coalition service member died following a separate explosion in southern Afghanistan, the coalition said. No further details were available.

So far this year, at least 524 U.S. 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.

Tuesday’s helicopter crash took place not far from Chanaryan village, but there was no damage to buildings, Rasoolyar said. U.S. and Romanian forces make up most of the NATO contingent in Zabul province.

“I was sitting taking my tea,” said Nakeemullah, 20, who transports livestock in the area and uses only one name. “I heard noise and I went outside to see what happened. I saw a lot of smoke in the sky. 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.”

Aircraft are used extensively in Afghanistan by both NATO and the Afghan government forces to transport and supply troops because the terrain is mountainous and roads are few and primitive.

Lacking shoulder-fired missiles and other anti-aircraft weapons, the Taliban rely mostly on machine guns and rocket-propelled grenades to fire at aircraft during takeoffs and landings.

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

On Aug. 16, 2005, two transport helicopters carrying NATO peacekeepers crashed in a western Afghan desert, killing 17 Spanish troops. The cause was found to be a combination of a “high risk” flight plan of flying close to the ground, adverse weather conditions and mountainous terrain.

Tuesday’s crash was the deadliest since May 2006, when a Chinook helicopter went down while attempting a nighttime landing on a small mountaintop in eastern Kunar province, killing 10 U.S. soldiers.

One of the heaviest single-day losses of life for allied forces in Afghanistan occurred on June 28, 2005, when 16 U.S. troops died aboard a Special Forces MH-47 Chinook helicopter shot down by insurgents in Kunar province.