A slow-moving brush fire was burning Monday night on rock-covered hills in eastern Simi Valley, and county fire officials said the blaze had already consumed 240 acres.
The fire broke out shortly before 2:30 p.m. about 200 yards north of the Ronald Reagan Freeway just east of Kuehner Drive, said Joe Luna, a spokesman for the Ventura County Fire Department.
Flames were moving east Monday night toward Rocky Peak Fire Road near the Ventura County-Los Angeles County line, Luna said.
The blaze had not been contained, and authorities were uncertain when the fire would be completely doused. But some crews were being released late Monday.
Massive rocks dotting the hills were slowing the fire's spread, authorities said.
It took three hours for the fire to triple in size from 50 to more than 150 acres, authorities said.
But the rugged terrain was also hampering efforts to fight the blaze from the ground. Luna said firetrucks could not drive into the heart of the fire and that firefighters were hiking in to construct a fire line, to stop the blaze from spreading.
Four water-dropping helicopters were battling most of the blaze from the air, Luna said.
"It's very rocky, hilly terrain, and there is a lot of work to do on the ground. We'll be here through the night and into morning," he said.
There was little or no wind most of the day, and the peak temperature was 83 degrees Monday afternoon.
Although there were no reports of injuries, firefighters were concerned about heat stroke and exhaustion. They consumed extra water and took frequent breaks, authorities said.
The cause of the fire had not been determined, but Luna said county arson investigators were called to the scene.
Authorities are pursuing leads, but Luna refused to elaborate except to say that the fire appears to have started at a trail head near Rocky Peak Fire Road.
Meanwhile, at the fire's peak Monday afternoon, large black clouds of smoke could be seen throughout Simi Valley and in Chatsworth in Los Angeles County, witnesses said.
As of 9 p.m., there had been no evacuations and the freeway remained open, authorities said. The fire was burning just east of a large community of homes.
A minor backup in the westbound lanes occurred about 6 p.m. as commuters slowed to watch the helicopters take turns dumping loads of water on the blaze.
Simi Valley and California Highway Patrol officers were directing traffic on surface streets to keep vehicles moving, Luna said.
The fire was being fought by 253 firefighters from Ventura and Los Angeles counties and the California Department of Forestry, Luna said.
There were 10 fire engines and seven hand crews, whose members were clearing brush and other unburned vegetation to slow the fire, Luna said.
The fire is the third largest this season in Ventura County. A brush fire two weeks ago burned 2,800 acres near Lake Piru, and a fire in late June at Kings Campground in the northeastern county blackened 650 acres.
Wolcott is a Times Community News reporter, and Metcalfe is a Times staff writer.