BANNING, CALIF. -- A huge wildfire in mountains east of Los Angeles that is Southern California's biggest blaze so far this year was still raging Monday, with thousands of people forced to evacuate their homes.

The blaze in Riverside County, among several wildfires across California, had consumed more than 41 square miles (about 106 square kilometres) of dry brush and timber since it broke out Friday evening, according to the California Department of Forestry and Fire Protection.

As of Monday morning, it was just 5 per cent contained and the fire along with coronavirus precautions made for added stress at an evacuation centre, said John Medina, an American Red Cross spokesman.

Volunteers used to "close contact" with evacuees have had to adjust their approach during a time of social distancing, Medina said.

"I mean, that's part of the recovery of a disaster, is that you have to show warmth and love and caring. And that's hard when you're standing six feet away. So that's the biggest challenge," Medina told KESQ-TV.

Bill Boehm helped his mother evacuate her home with her three horses and said the pandemic hasn't stopped people from coming together to assist each other.

"Everyone seemed to be wearing masks and such, so that type of precaution was still going on," Boehm told the news station.

Investigators are trying to determine the cause of the blaze that began as two adjacent fires in Cherry Valley, a rural area near the city of Beaumont, about 85 miles (137 kilometres) east of downtown Los Angeles.

Flames raced along brushy ridge tops and came close to houses while firefighters attacked from the ground and air.

One home and two outbuildings were destroyed, California fire officials said. No injuries were reported.

In Northern California, evacuations were ordered after a wildfire that sparked Sunday afternoon spread quickly and burned near homes near the East Park Reservoir in Colusa County. Complicating firefighters' efforts were temperatures in the area north of Sacramento were expected to hit the upper 90s (around 36 Celsius) by Monday afternoon.

On California's central coast, a fire in San Luis Obispo County was 60% contained Monday after burning more than 2 square miles (6 square kilometres) of brush east of the community of Santa Margarita. Two structures were destroyed but officials said it was unclear whether they were homes.

In Riverside County, officials allowed flames from the so-called Apple Fire to run up the side of Mount San Gorgonio, an 11,000-foot (3,350-meter) peak, because it wasn't safe to let crews work in such steep, rugged terrain, said Lisa Cox, a spokeswoman for the U.S. Forest Service.

Evacuation orders and advisories were issued for about 2,500 homes in mountain, canyon and foothill neighbourhoods. Campgrounds and hiking trails were closed in the San Gorgonio Wilderness area of the San Bernardino National Forest.

A huge smoke plume was visible for miles around and contributed to poor air quality.

The temperature hit 109 degrees (43 Celsius) Sunday in nearby Palm Springs but conditions were expected to cool slightly on Monday.