A blaze burning out of a control near Los Angeles sparked mandatory evacuations as it tore through swathes of tinder-dry California countryside around popular tourist spots.

The so-called Line Fire erupted on Thursday to the east of the city and exploded over the weekend, consuming more than 20,000 acres (8,000 hectares) and threatening tens of thousands of homes and other buildings.

Mandatory evacuation orders have been imposed and major roads shuttered, as more than 1,700 firefighters battle to contain the fast-moving flames.

Footage and photos from the scene show towering flames racing up the chaparral-covered hillsides.

Another blaze, north of Los Angeles, broke out on Sunday afternoon and had consumed 1,200 acres by Monday morning.

Firefighters ordered evacuations from the Bridge Fire as crews tackled it from the ground and from the air.

A punishing heatwave that has gripped the region since the middle of last week was exacerbating the problem, with high heat and gusty conditions making the spread of flames less predictable.

- People fleeing -

A much smaller fire in the north of the state was also causing concern, with 4,000 people fleeing the 76-acre Boyles Fire in Clearlake.

Firefighters said 30 buildings had already been affected by the fire and up to 50 vehicles had been destroyed.

In Nevada, the out-of-control Davis Fire has torn through 6,500 acres of heavy timber and brush since it started on Saturday.

At least 12,000 people in south Reno have been told to leave their homes, with more than a dozen buildings already known to have been damaged.

Footage on social media showed scores of wild horses seemingly running away from the blaze.

"Please watch out for each other, your friends, neighbors, loved ones. It is very critical in times of need," Reno Mayor Hillary Schieve said Sunday in a social media post.

- Heat warning -

Democratic Party vice presidential candidate Tim Walz cancelled a rally he had been due to hold in Reno on Monday evening because of the fire.

"We will continue to work with federal, state and local partners to monitor the situation, and we urge the impacted communities to heed the evacuation and safety guidance from local authorities," a campaign statement said.

Temperatures have repeatedly topped 110 Fahrenheit (43 Celsius) in several areas of southern California, including in parts of Los Angeles.

An excessive heat warning, which has been in place since the middle of last week, was not expected to be lifted until later Monday.

"Dangerous heat is in the forecast for at least one additional day for southern California and parts of the Southwest," the National Weather Service said.

"The combination of above average temperatures, dry vegetation, and gusty winds have resulted in elevated to critical fire weather concerns throughout much of the (area) through the next couple of days," it said.