(Updates with latest details on the fire; some in evacuation zone forced to stay put)
LOS ANGELES, Aug 8 (Reuters) - Firefighters gained ground against a wind-whipped California wildfire that has destroyed 15 buildings, injured at least five people and forced the evacuation of 500 homes in several small communities east of Los Angeles.
The fire erupted on Wednesday near a back-country road south of the town of Banning, and by nightfall on Thursday had blackened an estimated 14,000 acres (5,666 hectares), the Riverside County Fire Department said on its website.
Four firefighters and one civilian have been hurt in the blaze, which was raging through tinder-dry brush about 85 miles (137 km) east of downtown Los Angeles. The extent of their injuries was not immediately known.
As of Thursday evening, a firefighting force of roughly 1,000 personnel, backed up by more than a dozen water-dropping helicopters and several airplanes carrying payloads of fire-retardant chemicals, had managed to carve fire breaks enclosing 20 percent of the blaze.
That figure was double the containment estimate issued at the start of the day.
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 town of 2,500 residents about 20 miles (32 km) west of the desert resort 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 were under evacuation orders, Berlandt said. Shelters for evacuees had been set up at high schools in nearby Hemet and Beaumont.
A number of residents living within the evacuation zone were initially advised to remain inside their homes as approaching flames cut them off from a safe escape route, county fire spokeswoman Melody Hendrickson said. Those evacuees have since been moved out of harm's way.
At least 15 structures had been destroyed by the fire, but damage assessment teams were still taking inventory of the property losses to determine how many of the burned buildings might have been residences.
"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 was one of the latest to break out during what experts say could become one of the worst U.S. fire seasons on record.
A Colorado wildfire in June, ranked as that state's most destructive ever, ravaged nearly 500 homes and killed two people. In Arizona, 19 members of an elite firefighting crew died on June 30 while battling a wildfire. (Reporting and writing by Steve Gorman; additional reporting by Tim Gaynor; Editing by Cynthia Johnston, G Crosse, Vicki Allen and Ken Wills)