The Ventura County Fire Department said more than 200 firefighters were at or on the way to the fire, which began near the Conejo Grade of the freeway about 6:30 a.m. At 8:15 a.m., officials said additional air resources were being brought in as the fire was uncontained.
"Firefighters are working in steep, rocky terrain to stop the spread of the fire," according to a Ventura County Fire update.
At least one southbound lane of the freeway was closed. The Fire Department said Old Conejo Road was open only to residents.
Ventura and L.A. counties remained under a red-flag warning through Friday night, with humidity expected to be extremely low and hot winds blasting north across the region at speeds upward of 40 mph in the valleys and coasts and 70 mph in the mountains.
The weather is fanning the Summit fire in Riverside County, which scorched about 3,000 acres since Wednesday afternoon. Two firefighters have been injured and one building damaged in that fire.