Hundreds of firefighters enduring high temperatures are battling against a wildfire along a major highway, as the western US broils under another day of blistering temperatures.

"Excessive heat, low humidity and steep terrain will continue to pose the biggest challenge for firefighters," authorities say. (AP)

California firefighters have worked in extreme conditions as they battled wildfires in rural areas north of Los Angeles and east of San Diego amid a blistering heatwave that is predicted to last through Labor Day.

Seven firefighters had to be taken to hospitals with heat injuries, said Los Angeles County Fire Department Deputy Chief Thomas Ewald on Thursday. 

All were released, Ewald said, adding that he expected more heat emergencies.

Progress was made in containing both blazes but authorities warned that 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 percent contained after scorching more than 21 sq km 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 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 the use of electricity during critical afternoon and evening hours.

"Reducing energy use during a Flex Alert can help stabilise the power grid during tight supply conditions and prevent further emergency measures, including rotating power outages," California Independent System Operator (ISO) said.

READ MORE: 'Dangerous' heatwave grips southwestern US

READ MORE: California reports multiple fatalities from ongoing wildfire

Authorities issued an
Authorities issued an "excessive heat warning" for most of California, as well as parts of Arizona and Nevada, warning of "dangerously hot conditions" over the next several days. (AP)

'Multiple close calls'

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, Ewald said. "The big thing today is all about boxing the fire in," he said.

In eastern San Diego County, the Border 32 Fire was five percent contained after swiftly growing to more than15.5 sq km, 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. State routes in the area were closed and a school district called off Thursday's classes.

There were "multiple close calls" as residents rushed to flee, said Cal Fire Capt. Thomas Shoots.

"We had multiple 911 calls from folks unable to evacuate" because their homes were surrounded by the fire, Shoots told the San Diego Union-Tribune.

US Customs and Border Protection closed the Tecate port of entry with Mexico hours early on Wednesday night because of the fire and said it wouldn't reopen until conditions improved. Travellers could still use the 24-hour Otay Mesa crossing.

Wildfires have sprung up this 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 the climate crisis 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.

READ MORE: California: Largest blaze of the year claims lives as thousands evacuate

Source: TRTWorld and agencies