San Francisco  - Nine firefighters died when a helicopter carrying them to help battle a wildfire in northern California crashed and burned in a remote forest area, the Federal Aviation Authority said Wednesday.

Four other people on board the Sikorsky S-61 military-style helicopter suffered burns in the crash and were hospitalized in serious to critical condition.

The FAA said that the helicopter crashed under "unknown circumstances" as it was being used as a fire transport vehicle to attack the Buckhorn Fire in the Trinity-Shasta National Forest, helping to ferry firefighters around the blaze.

The Sikorsky S-61 helicopter is the only wildland firefighting helicopter in the nation equipped to simultaneously carry both water and crew. It can simultaneously carry up to
18 firefighters and drop up to 4,000 litres of water via a suspended bucket. (dpa)