A fast-moving wildfire advanced to the outskirts of the Northern California city of Chico early on Friday, forcing thousands to flee.
It had already left the nearby town of Paradise in ruins.
Fire officials issued a mandatory evacuation notice for homes east of Chico, a city of about 93,000 people situated about 145 km north of Sacramento.
Flames from the unchecked, 8,100-hectare Camp Fire were being driven westward by 55kph winds.
The fire earlier ripped through Paradise, about 30 kilometres east of Chico.
"The town is devastated, everything is destroyed. There's nothing much left standing," said California Department of Forestry and Fire Protection (Cal Fire) spokesman Scott Maclean.
"This fire moved so fast and grew so fast a lot of people got caught by it."
Mr Maclean said a number of civilians and firefighters had been injured, and that it could be days before authorities would know whether anyone had died.
Paradise is located on a ridge and had limited escape routes. Traffic accidents turned roads into gridlock and residents abandoned vehicles and ran from the flames carrying children and pets, officials said.
One woman stuck in traffic went into labour, the Enterprise-Record newspaper reported.
"It’s very chaotic,” Officer Ryan Lambert of the California Highway Patrol.
Rescuers used a bulldozer to push abandoned cars out the way to reach Feather River Hospital and evacuate patients as flames engulfed the building, Butte County Supervisor Doug Teeter said.
The hospital was totally destroyed, Mike Mangas, a spokesman for operator Dignity Health, told Action News Now.
The fire, which began early on Thursday, was the fiercest of several wind-driven blazes across California, during what has been one of the worst years for wildfires in the state.