A wind-whipped California wildfire that doubled in size overnight torched 15 buildings east of Los Angeles, injured at least five people and forced the evacuation of 500 homes in about half a dozen small communities, authorities said on Thursday.

The fire broke out on Wednesday near a back-country road in Riverside County, and by early Thursday had blackened more than 10,000 acres, the Riverside County Fire Department said on its website.

Four firefighters and one civilian have been hurt in the blaze, which is raging through tinder-dry brush and is just ten percent contained. The extent of their injuries was not immediately known.

Vista Grande, Mount Edna, Poppet Flats, Twin Pines and Silent Valley were among several communities under mandatory evacuation orders as the fire burned toward Cabazon, a city of 2,500 residents about 20 miles west of Palm Springs.

"The dry conditions right now that we are seeing are allowing the fire to burn very quickly, then you add the gusting winds ... and it is pushing the fire further and further to the east," California Department of Forestry and Fire Protection spokesman Daniel Berlandt said.

About 500 homes are under evacuation, Berlandt said. Shelters for evacuees had been set up at high schools in nearby Hemet and Beaumont.

At least 15 structures had been destroyed by the fire, although it was not immediately clear whether any of them were homes.

"We are still working to confirm what type of structures and how many were burned. We believe there were 15 structures, but we don't know if they were barns, outbuildings or homes," Berlandt said.

Authorities have not yet determined how the fire started.

The blaze is the latest to break out during what experts say could become one of the worst U.S. fire seasons.

A Colorado wildfire in June, ranked as that state's most destructive on record, ravaged more than 500 homes and killed two people. In Arizona, 19 members of an elite firefighting crew died on June 30 while battling a wildfire.