Nine people are feared dead in a helicopter crash in the US state of California.

The helicopter was travelling to a wildfire when it crashed, a Federal Aviation Administration spokesman confirmed.

Another four aboard the helicopter were critically burned in the crash approximately 56km northwest of Redding in remote terrain.

The helicopter, with a two-person crew and carrying 11 firefighters, crashed under unknown circumstances, the spokesperson said.

FAA and National Transportation Safety Board investigators are on the way to the scene of the crash.