Tankers, several helicopters and more than 150 firefighters were on scene to stave off flames that were creeping toward several large hillside homes. The blaze began about 11:30 a.m. in Murrieta, about 90 miles southeast of Los Angeles in Riverside County.
Television images showed plumes of grey smoke blanketing hillsides and the fire approaching at least two large-scale homes.
There have been no evacuation orders and no reports of any injuries.
No containment estimates were given.