Residents speak of their fears as the fires continue to spread
Firefighters tackling an intense wildfire north of Los Angeles say the blaze remains "very dangerous".
Fuelled by hot weather and dense, dry brush, the blaze doubled in size in a day to spread over 134 sq miles (347 sq km) of forest.
About 12,000 buildings and key communications masts are under threat, and officials say the fire could easily double again over the coming days.
Two firefighters died on Sunday after their vehicle was overrun by flames.
Inspector Frederick Stowers of LA County Fire Department told the BBC that only 5% of the fire had so far been contained.
He said it could take until after the next weekend to bring the blaze under control.
"We are working around the clock and using every resource we can," Insp Stowers said.
More than 10,000 residents have fled the flames, and some 6,600 homes are under mandatory evacuation orders.
At least 18 houses have been destroyed, and that number was expected to grow, officials said.
The fires also threaten communication towers housing transmitters for Los Angeles' major TV stations.
Mount Wilson, where the towers are located near the city of Pasadena, is also home to a historic solar observatory.
Mike Dietrich, of LA County Fire Dept, said: "We are making progress. But it is... slow and very dangerous. We have to wait for the fire to come to us."
Firefighters are on the mountain clearing brush and spraying fire retardant in preparation for the fires, which are approaching the transmitters.
Fire department officials say the area's steep, rugged hills are making it difficult for the 3,600 firefighters.
"It's burning everywhere," US Forest Service spokeswoman Dianne Cahir told Associated Press news agency.
"When it gets into canyons that haven't burned in numerous years, it takes off."
California Governor Arnold Schwarzenegger: We have "fires, fires, fires"
The Governor of California, Arnold Schwarzenegger, has declared a state of emergency and urged people to comply with evacuation orders.
Wildfires are a feature of the Californian summer, but it is unusual for them to break out so close to major population centres.
The latest fire broke out on Wednesday and its cause remains under investigation.
But it is not being fanned by the Santa Ana winds that typically kick up in October. Instead, it is being fuelled by extremely dry brush that has not burned in more than 40 years.
A number of other fires are also burning in southern and central California.
A new fire in Placer County, north-east of the state capital, Sacramento, has destroyed 60 structures, many of them homes in the town of Auburn.
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