Jim Miklaszewski, Courtney Kube and Fazul Rahim, NBC News posted:

KABUL - Six U.S. soldiers serving with the NATO International Security Assistance Force were killed when their Sikorsky UH-60 'Black Hawk' helicopter crashed in southern Afghanistan Tuesday, officials said.

The cause of the crash is under investigation, NATO said in a statement, but a senior U.S. defense official said the aircraft had a hard landing due to mechanical problems and was attacked by militants after it went down.

NATO did not disclose the nationalities of the victims but the U.S. official said they were American.

There were no reports of enemy fire in the area the when helicopter went down, the NATO statement added.

The helicopter crashed in the Shahjoi district of the Zabul province, according to its deputy governor Mohammad Jan Rasulyar.

Aircraft crashes are not uncommon in mountainous Afghanistan. In August 2011, 25 U.S. special operations forces were among 38 killed when the Taliban shot down a transport helicopter.

About 84,000 NATO-led troops are serving in Afghanistan,including about 60,000 from the United States. That number is expected to be reduced to about 10,000 by 2015.