NATO says 11 people, including seven American troops, have been killed in a helicopter crash in southern Afghanistan.

The other casualties, according to a statement from the international military coalition, were "three members of the Afghan National Security Forces, and one Afghan civilian interpreter."

It was not immediately clear what caused the helicopter to crash. ISAF said an investigation was underway. The Taliban issued a statement claiming that militants had shot the aircraft down, but the group often issues claims which later turn out to be false or exaggerated.

Afghan officials told the Reuters news agency the crash took place in the Shah Wali Kot district of Kandahar province. Kandahar has been a stronghold for Taliban militants an has seen a large proportion of the violence directed at Western and Afghan forces since the U.S. invasion to topple the group's government in 2001.

Taliban militants and their allies in the region have stepped up their attacks during August - an annual uptick which coincides with fair weather and the Muslim holy month of Ramadan.