The fire was first reported at 1:20 p.m. near Potrero Road and Wolfskill Truck Trail. By 5:30 p.m., it had grown to 1,100 acres, and it had nearly doubled in size to 2,000 in little more than an hour.
Officials said it was 10 percent contained, but there was no estimate for when they would have it under control.
There were no injuries reported, structures in danger or evacuations.
Officials said the fire was burning in steep, rugged terrain and gusty conditions were driving the fire. Gusts of up to 30 mph were reported.
About 280 firefighters, including from CAL FIRE, the Riverside County Fire Department and San Diego County, were at the scene. Officials said they would remain there through the night.
The cause of the fire was being investigated.