A NATO helicopter crashed into a house on the outskirts of the Afghan capital, Kabul, on Friday, killing 12 Turkish soldiers on board and four Afghan civilians on the ground, Turkey’s general staff and a senior Afghan police official said.

The crash came amid growing unease among NATO partner countries about the increasingly unpopular and costly war nearly 11 years into the conflict as most foreign combat troops set to leave Afghanistan by the end of 2014.

“Twelve of our military personnel on board the helicopter have been martyred,” the Turkish General Staff said in a statement in Ankara. A team had been sent to the scene of the crash to investigate the cause, it said.