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.

In a brief official announcement, the American military command in Kabul said there were no confirmed reports of casualties at this time. The announcement did not specify the nationality of the helicopter crew and said the cause of the crash was unknown.

Two U.S. defense officials told the Associated Press four U.S. troops were aboard the helicopter, identified as an Army Black Hawk, and one official said initial word from the scene was that officials don't expect that any of the four survived. Both officials spoke on condition of anonymity because details were still being collected and the families of the helicopter crew had not yet been notified.

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.