Updated 6:37 p.m. ET
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.
Military officials confirmed to CBS News national security correspondent David Martin that all killed were American troops. The helicopter involved in the crash was a Blackhawk UH-60, a workhorse of the U.S. armed forces.
One other coalition service member, an Afghan National Army soldier and a U.S. civilian were wounded.
Sources tell CBS News correspondent Mandy Clark the helicopter was flying a covert Special Operations mission, which are often undertaken at night to take advantage of the cover of dark. The missions are often aimed at disrupting Taliban activities by hunting high-value commanders, or finding drug or weapons caches.
Covert, nighttime operations aimed at high-value targets are a key part of the war strategy, Clark reports. U.S. forces have been swarming into the outskirts of Kandahar in an effort to flush the Taliban out of its traditional stronghold.
With the surge now complete, American troops are going on more missions. That means more risk and more casualties, CBS News Security Analyst Juan Zarate reports.
"The downing of a helicopter, whether it was accident, weather related or enemy related, to a certain extent is to be expected when you have more people, more soldiers, more helicopters in that region," Zarate said.
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 told The Associated Press by telephone that insurgents shot down the helicopter. The Taliban often exaggerate their claims and sometimes take credit for accidents.
"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. "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."
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.
One of the heaviest single-day losses of life for allied forces in Afghanistan took 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.
Afghan troops have been killed in helicopter crashes as well.
In January 2009, a top Afghan Army general for the western region of Afghanistan and 12 others were killed when their MI-17 helicopter went down in Shindand district of Herat province.
Also on Tuesday, suspected U.S. missiles struck on both sides of the Pakistan-Afghanistan border, killing at least 19 alleged militants, according to three Pakistani intelligence officials. They said seven of the deaths were in Afghanistan and 12 in Pakistan. The second strike hit the compound in Pakistan where bodies from the first strike were taken.
In Afghanistan, NATO said it had no reports of such an incident. U.S. officials rarely discuss the covert CIA-led missile program.
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 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.