(Adds survivor, updates to show aircraft was a helicopter, adds new troop figures)
KABUL, Dec 17 (Reuters) - Six U.S. soldiers were killed in a helicopter crash in southern Afghanistan on Tuesday, NATO said, the largest death toll in a single incident to hit the international force in months.
"The cause of the crash is under investigation; however, initial reporting indicates there was no enemy activity in the area at the time," a NATO statement said.
U.S. defense officials said the soldiers killed in the crash, which took place in Afghanistan's southern Zabul province, were American. One person survived the crash but suffered injuries.
Aircraft crashes are not uncommon in mountainous Afghanistan.
The worst such incident was in August 2011 when the Taliban shot down a transport helicopter, killing all 38 people on board including 25 U.S. special operations soldiers.
The Pentagon said that about 67,000 NATO-led troops remain in Afghanistan, including about 43,000 from the United States. (Reporting by Dylan Welch in Kabul, and David Alexander and Missy Ryan in Washington; Editing by Missy Ryan and Jonathan Oatis)