The US Army is sending hundreds of troops to help tackle wildfires raging across parched California, authorities said Tuesday, while also warning of the growing risk posed by drone pilots flying over affected areas.

Navy Captain Jeff Davis, a Pentagon spokesman, said 200 troops from Washington state would receive three days' training before heading to California to help the 12,000 or so firefighters in the region.

"This is the first time since 2006 that (the Defense Department) has mobilized federal active duty military personnel" to fight wildfires, Davis said.

Eighteen wildfires are currently raging in California, and 10 states around the country are reporting large fires.

A heatwave is exacerbating four years of drought and making California even more vulnerable.

State fire agency CAL FIRE cautioned of a growing problem of people flying hobby drones over affected areas.

"A collision with a hobby drone could easily result in major damage to firefighting aircraft, injuries to the pilot... or worse, a midair collision," the agency said in a statement. "If you fly, we can't!"

The Pentagon has already deployed four firefighting air tankers to help in efforts.

Another 440 soldiers and airmen from the California National Guard have also been mobilized.

Temperatures are expected to cool to more normal seasonal levels starting Wednesday in Southern California, but high winds will continue to threaten the spread of blazes.