Hundreds of firefighters battled raging wildfires in the western United States on Tuesday amid a record-setting heatwave that has claimed at least four lives and triggered evacuations.

More than 600 firefighters and water-dropping helicopters worked to contain two major brush fires that erupted east of Los Angeles on Monday, scorching more than 5,400 acres (2,185 hectares) and forcing about 850 families to flee their homes.

There were fears that the two fires in the Angeles National Forest and foothills above Duarte and Azusa -- about two miles (3.2 kilometers) apart -- could merge creating an huge inferno.

One of the fires was sparked by a fatal car crash.

"I came running over just to look and it was 15 to 20 feet in the air," Charlie Downing, a resident in Duarte, told reporters as he recounted rushing out of his home on Monday after he smelled smoke.

"By the time I came back and told my grandma and my kids to get in the car, it was right by the car."

- More evacuations, more heat -

The fire eventually shifted away from residential areas late Monday but officials said they remain on alert as the flames could quickly change direction depending on the wind.

They added that both fires, which are threatening wilderness in the area, had yet to be contained early Tuesday and more evacuations could be ordered.

Elsewhere, officials reported that a fire that has raged for nearly a week in mountains west of Santa Barbara, California, was 62 percent contained after scorching some 8,000 acres.

The heatwave, which has resulted in triple-digit temperatures of up to 122 degrees Farenheit (50 degrees Celsius), has resulted in 21 large fires in nine states, including Colorado, Montana and New Mexico.

In Arizona, at least four people, including a German hiker and a 28-year-old biker, succumbed to the heat on Sunday.

The National Interagency Fire Center said Tuesday that several new large fires had been reported in California and Colorado and the intense heat was set to continue across much of the Southwest.

The National Weather Service said it expected temperatures to remain high in many regions on Tuesday, including in areas around Los Angeles where red flag warnings of extreme heat were issued.

Authorities urged people in those regions to remain indoors, to drink a lot of water and avoid strenuous outdoor activity.

The five-year drought in California and the scorching weather have created burning conditions more typical of August, the US Forest Service said, warning that the worst was maybe yet to come.

Wildfires across the United States scorched a record 10 million acres in 2015, according to the National Interagency Fire Center.

The largest fires were all in Alaska but two fires in California -- the Valley and Butte fires -- rank among the 10 most destructive in the state's history.