Crews face heatwave in California fires

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California firefighters are working in extreme conditions as they battle wildfires in rural areas north of Los Angeles and east of San Diego amid a blistering heatwave.

Progress was made in containing both blazes on Thursday but authorities warned the explosive fire behaviour that occurred after they erupted on Wednesday showed the potential for what could happen during the prolonged torrid conditions.

"The days ahead are going to be challenging," said Angeles National Forest Fire Chief Robert Garcia, one of the commanders of the battle against the Route Fire near the Interstate 5 community of Castaic in northwestern Los Angeles County.

The Route Fire was 12 per cent contained after scorching more than 21 square kilometres and destroying a house. Traffic on the major north-south interstate, a key route for big rigs, was jammed due to lane closures.

Temperatures in the area hit 42C on Wednesday.

Seven firefighters were taken to hospital with heat injuries.

Temperatures in much of California were so high that Governor Gavin Newsom declared a state of emergency and the state power grid operator asked residents to voluntarily reduce use of electricity during critical afternoon and evening hours.

After strong work by ground crews and helicopters and airplanes dropping water and fire retardant on the Route Fire, authorities planned to lift evacuation orders for a mobile home park and other homes.

In eastern San Diego County, the Border 32 Fire was five per cent contained after swiftly growing to more than 16 square kilometres, the California Department of Forestry and Fire Protection said.

The fire burned at least four buildings, including a house, and prompted evacuations for some 400 homes in the Dulzura area near the US-Mexico border.

Wildfires have sprung up this northern summer throughout the Western states. The largest and deadliest blaze in California so far this year erupted in July in Siskyou County. It killed four people and destroyed much of the small community of Klamath River.

Scientists say climate change has made the West warmer and drier over the last three decades and will continue to make weather more extreme and wildfires more frequent and destructive.