Wildfires have spread rapidly through Northern California, destroying hundreds of homes, forcing thousands of people to flee and injuring four firefighters.

Driven by high winds and soaring temperatures, the Valley Fire in southern Lake County has grown to 25,000 acres (10,117 hectares), CAL FIRE spokesman Daniel Berlant said on Twitter.

The Butte Fire in Amador and Calaveras counties is now burning through 65,000 acres and is just 15 percent contained, according to CAL FIRE.

Nearly 4,000 firefighters are battling that blaze, which has destroyed 86 homes and 51 outbuildings so far.

Wildfires sweeping across the drought-stricken western United States this year could be the costliest on record.

Berlant told AFP that CAL FIRE, California's firefighting agency, has so far spent more than $212 million since July alone and about 5,000 firefighters are still assigned to major wildfires. 

National Guard troops who had been assisting the firefighting effort have been released.

More than 275 homes and other buildings have been destroyed in the recent fires, Berlant said, adding that "thousands" of residents have been evacuated.

"The Red Cross has opened emergency shelters to provide evacuated residents a place to go," he said.

The four injured firefighters suffered second degree burns and were in stable condition at UC Medical Davis Center.

Officials say across the western United States, $1.23 billion have been spent so far this year, and 30,000 firefighters have been mobilized.

The all-time record for firefighting costs in the United States -- $1.65 billion -- was set in fiscal 2002.