Firefighters in California are battling several new blazes that ignited as the state is sweltering under a grueling heatwave.

Flames burned through bone-dry brush in rural areas north of Los Angeles and east of San Diego on Thursday, forcing the closure of a major interstate and prompting evacuations of schools and care facilities.

The new fires come amid a prolonged and possibly record-setting heatwave that is engulfing the western US and is expected to last until Labor Day. Temperatures in the mid to upper 90s and lower 100s will probably result in widespread daily temperature records each day, the National Weather Service said on Thursday.

Excessive heat warnings are in effect across the desert south-west and California, with heat advisories stretching across much of the central Great Basin.

High night-time temperatures will only increase the heat stress, the service warned, creating potentially dangerous situations for vulnerable individuals.

Some areas in California already broke records – on Wednesday, Burbank reported a new August high at 112F.

The state’s governor, Gavin Newsom, declared a state of emergency in response to the high temperatures, and warned the heatwave will strain the state’s power grid. The operator of the grid has asked residents to reduce use of electricity to avoid forced outages.

Meanwhile, parents in Los Angeles have called for more shade at local schools, where playground asphalt can reach temperatures of 145F, according to the Los Angeles Times.

The heat has created particularly difficult working conditions for firefighters. Seven firefighters suffered heat injuries while battling the Route fire near Castaic and had to be treated at hospitals, said Thomas Ewald, the Los Angeles county fire department deputy chief.

Temperatures in the area on Wednesday hit 107F (42C) and winds gusted to 17mph (27 km/h), forecasters said. More heat emergencies are expected, Ewald added, as crews grapple with the extreme weather that is expected to linger until next week.

“Wearing heavy firefighting gear, carrying packs, dragging hose, swinging tools, the folks out there are just taking a beating,” he said.

The Route fire in north-western Los Angeles county destroyed a home after raging through more than 8 sq miles of hills containing scattered houses. The blaze briefly closed Interstate 5, a key route for big rigs, before authorities reopened some lanes on the highway on Thursday morning. Traffic has remained snarled as firefighters try to get a hold on the fire, which currently is estimated to be 12% contained.

“The days ahead are going to be challenging,” said Robert Garcia, the Los Angeles national forest fire chief, of the battle against the Route fire.

Media reports showed a wall of flames advancing uphill and smoke billowing thousands of feet into the air while aircraft dumped water from nearby Castaic Lake. Authorities planned to lift evacuation orders for a mobile home park and other homes in the area thanks to strong work from ground crews, helicopters and airplanes dropping water and fire retardant on the blaze, Ewald said.

A separate fire in eastern San Diego county, near the US-Mexico border, burned at least four buildings and prompted evacuations.

The fire swiftly grew to more than 6 sq miles and prompted evacuation orders for at least 400 homes, authorities said. The fire was 5% contained.

No injuries were immediately reported, but there were “multiple close calls” as residents rushed to flee, said Capt Thomas Shoots of the California department of forestry and fire protection (CalFire).

“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 announced that the Tecate port of entry with Mexico closed three hours early on Wednesday night because of the fire and would not reopen until conditions improved to ensure “the safety of the traveling public”.