The fire started around Sunday in the Hathaway Canyon, northwest of Banning, the Los Angeles Times reported.
No structures are threatened by the blaze, authorities said. No injuries were reported.
About 1,000 firefighters have been brought in the battle the fire.
On Sunday in Los Angeles County, about 40 firefighters fought to quell small brush and grass fires in Gorman and Lake View Terrace. Those fires were quickly brought under control, said Ed Pickett of the Los Angeles County Fire Department.