Thousands of people evacuated their homes as a "rapidly-moving" wildfire engulfed the Sierra foothills in Northern California, authorities said Thursday.

Acting California Governor Gavin Newsom called a state of emergency in Butte County, north of Sacramento, after the so-called Camp Fire grew to more than 18,000 acres in just eight hours with none of it contained.

"We have received reports of some fatalities, those reports have not yet been verified," Butte County Sheriff Kory Hones told a press conference.

"This was a rapidly-moving fire. It moved very very quickly from a rather remote area into populated areas," said Hones, adding that the fire is still "very active."

"There was significant structure loss in multiple communities... as well as multiple civilian and firefighter injuries," said California Department of Forestry and Fire Protection (CalFire) Chief Darren Read.

The Butte County Sheriff's office said an evacuation order was issued for areas of Paradise, a town of about 27,000 people.

"This fire is very dangerous, please evacuate if asked to do so!" CalFire tweeted.