Authorities scramble for resources to beat back flames as more than 12,000 homes in western state are threatened.
A state of emergency has been declared in the northern areas of the US state of California as a fast-expanding wildfire injured four firefighters and forced thousands of people to evacuate small towns.
California Governor Jerry Brown issued the order on Sunday as mandatory evacuations were carried out in an area of about 3,000 residents.
The injured firefighters were hospitalised with second-degree burns after they were dropped off by helicopter to build containment lines against the so-called Valley Fire, which broke out on Saturday in Lake County, north of San Francisco .
The blaze with zero containment raced through several towns overnight after exploding in size within hours in hot weather and drought conditions.
An Associated Press photographer reported seeing dozens of homes destroyed and a complex with roughly 40 apartments burned to the ground.
State authorities also said at least 5,000 people were without power, and several residents said they also had no running water.
Fire officials were not able to confirm the number of structures burned.
The Valley Fire, with 1,000 personnel assigned to fight it, is the latest in a string of large and destructive wildfires that have ripped through drought-stricken brush and forest on the West Coast of the US over the summer.
Firefighters were battling 13 active fires in California including the 40,000-acre Valley Fire, which was zero percent contained and the 65,215-acre Butte Fire, which was 20 percent contained on Sunday.
Governor Brown previously declared an emergency in two rural areas, where the Butte Fire began on Wednesday, destroying 86 homes and forcing the evacuation of thousands of residents since.