The fire is raging westward through largely undeveloped foothills of the San Bernardino Mountains, but it is dangerously close to subdivisions to the south.
County fire spokeswoman Jody Hagemann says at least 425 firefighters are working to gain control of the fast-moving fire that broke out Wednesday and has destroyed one structure.
Evacuations and street closures are in effect in several neighborhoods to the south of the fire in Banning.
If winds from the east continue driving flames westward, the fire could affect communities in Cherry Valley and Beaumont.
Much of Southern California is under red flag warnings for fire danger due to winds, low humidity and heat.