A Black Hawk NATO helicopter has crashed in the southwestern region of Afghanistan, carrying four people on board, according to reports.

It is not clear if anyone survived.

All passengers are believed to be American nationals.

A U.S. defense official told BBC that it appears poor weather was the culprit, not the action by opposing militants.

However, Reuters reported that the area where the helicopter went down includes the traditional Taliban stronghold of Helmand province.

BBC also said the crash site is secure and the crash is under investigation.

The U.S. and its allies are scheduled to depart Afghanistan by the end of 2014.

Nearly 3,000 foreign soldiers have died in Afghanistan since the war against the Taliban began in 2001.