Six NATO troops died Tuesday when an aircraft crashed in southeastern Afghanistan, officials said.

The crash, which occurred in Zabul province, according to deputy provincial governor Mohammad Jan Rasoulyar, marked the single deadliest incident of the year for the coalition. In a statement, the U.S.-led International Security Assistance Force (ISAF) said the “cause of the crash is under investigation, however initial reporting indicates there was no enemy activity in the area at the time of the crash.”

The Taliban, meanwhile, asserted responsibility on Twitter, writing that its fighters shot down a helicopter while it was “flying at low altitude” over Zabul.

Earlier this year, six Americans, including three civilians, were killed in a suicide blast in the province.

The ISAF did not disclose the nationalities of the victims, but the bulk of troops in southern Afghanistan are American.

The United States is in the midst of a rapid drawdown in Afghanistan. As of February, 32,000 American troops will remain in Afghanistan, a number that will decline steadily through the end of 2014.