KABUL, Afghanistan — A NATO helicopter crashed Tuesday in southern Afghanistan, killing nine international troops in a region where forces are ramping up pressure on Taliban insurgents. It was the deadliest chopper crash for the coalition in four years.

A "large number" of Americans were among those who died, according to a senior military official in Washington, who spoke on condition of anonymity because not all relatives had been notified. He said it remained unclear whether troops of other nationalities were among the fatalities.

The cause was not immediately clear. The Taliban 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 U.S. civilian were injured.

So far this year, 525 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.

The helicopter crash occurred in northwestern Zabul province in the early morning hours, according to a NATO official, who spoke on condition of anonymity because he was not authorized to disclose the location of the crash. 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 U.S. 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, Taliban spokesman Qari Yousef Ahmadi told The Associated Press by telephone that insurgents shot down the helicopter. The Taliban often exaggerate their claims and sometimes take credit for accidents.

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

"The Taliban were not involved in this crash at all," he said.

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

The worst helicopter crash for coalition forces was in August 2005 when two transport choppers clipped rotors and crashed in a western Afghan desert, killing 17 Spanish troops.

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

Meanwhile, suspected U.S. missiles struck on both sides of the Pakistan-Afghanistan border Tuesday, killing at least seven alleged militants, three Pakistani intelligence officials said. The second strike hit the compound in Pakistan where bodies from the first strike were taken, they said.

NATO in Afghanistan said it had no reports of such an incident. U.S. officials rarely discuss the covert CIA-led missile program.

The first strike hit an insurgent vehicle in the Khand Narai area of Paktika province in eastern Afghanistan, killing the seven. Two of the three Pakistani intelligence officials said the militants 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.

The officials said the militants had just crossed into Afghanistan when the missiles hit. Their bodies were taken across the border to Azam Warsak village in Pakistan’s South Waziristan tribal region. A second strike hit the compound in that village later Tuesday evening. It was not immediately clear if anyone died in the second strike.

The Pakistani officials spoke on condition of anonymity because they were not authorized to speak to media on the record.

The U.S. routinely uses missile strikes to take out insurgent groups. Most hit targets in Pakistan’s tribal belt, a semiautonomous and lawless stretch of territory that has a very porous border with Afghanistan.

Also on Tuesday, five Afghan road construction workers were killed and four wounded when their vehicle hit a roadside bomb in Shinwari district of Parwan province, said Shinwari police official Abdul Shakoor. Parwan is in northeastern Afghanistan.

The violence follows a spate of attacks around the country as it held parliamentary elections on Saturday. Officials said militant attacks on election day killed at least 21 civilians and nine police officers.

Separately, the Afghan Electoral Complaints Commission has received nearly 3,000 formal complaints regarding Saturday’s polls, including allegations of intimidation, ineligible ballots, misuse of voter registration cards, proxy voting and shortages of ballot papers, according to the U.S.-based Democracy International, which has been observing the vote.

The deadline for filing complaints was Tuesday, but the commission was considering a two-day extension, the observer group said in a statement. Most of the allegations were made before the polls closed.

Results of the elections could take weeks or even months to compile.

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Associated Press writers Mirwais Khan in Kandahar, Ishtiaq Mahsud in Dera Ismail Khan, Pakistan, and Anne Gearan in Washington contributed to this report.