A white transport helicopter crashed in southern Afghanistan Tuesday, killing at least six on board, said NATO spokesman. 

The helicopter went down in the Sangin district of the southern Helmand province. It's not immediately clear what caused the crash.

"At around 9 a.m. this morning, a private helicopter has crashed outside Sangin military base.… We have at least six people killed," a spokesman for the NATO-led International Security Assistance Force told the Agence France-Presse news agency.

He said the passengers were all civilians. Their nationalities aren't immediately known.

The helicopter was ablaze before it crashed, Sangin's district chief Fazlul Haq told Reuters.

It's not immediately clear who owns the helicopter, said U.S. military spokeswoman Lt. Cmdr. Christine Sidenstricker.

White transport helicopters are used by private contractors and organizations such as the UN and the U.S. Embassy, but UN and U.S. officials said the helicopter did not appear to be one of theirs.

British and U.S. forces recently began a large-scale military offensive in Helmand, considered the heartland of the Taliban and the world's largest opium poppy producing region.

With files from The Associated Press