Tens of thousands of people in southern California were ordered to flee a raging wildfire burning out of control in a rural mountain area on Wednesday, authorities said.

The Blue Cut fire in Cajon Pass, about 100 kilometres east of Los Angeles in San Bernardino County, mushroomed into an inferno covering more than 121 square kilometres, the National Wildfire Coordinating Group said.

More than 82,000 people were given a mandatory evacuation order in the face of what authorities called an "imminent threat to public safety, rail traffic and structures."

News videos showed houses on fire and at least one local business, a well-known roadside diner, has been destroyed in the blaze, according to the Los Angeles Times.

Amid desperate efforts to ensure evacuations and control the fire there was no official tally of property damage, but authorities spoke of dozens of homes destroyed.

"There will be a lot of families that come home to nothing," San Bernardino County Fire Chief Mark Hartwig told media.

The fire closed multiple roads, including a major interstate highway linking Los Angeles and Las Vegas.

The cause of the fire that started alongside the I-15 interstate highway Tuesday morning remains unknown.

But firefighter veterans said hot weather, drought-dry brush, and rugged terrain which has funnelled winds and thwarted firefighting efforts, have combined to create ideal conditions for a firestorm.

"In my 40 years of fighting fires, I have never seen such extreme fire behaviour," fire incident commander Mike Wakoski told Southern California Public Radio.

More than 1,300 firefighters were battling the fire but had yet to gain control, authorities said.

Air-tankers and helicopters swept the flames from above as ground crews from more than 150 fire trucks fought the blaze on the ground. Two firefighters were injured Tuesday when a firefighting team was trapped by flames.

More hot, dry, windy weather was expected to fan the flames at least through Thursday.

California Governor Jerry Brown Tuesday declared a state of emergency for San Bernardino County, saying the fire was too big for the region to fight on its own.

Brown linked the severity of the blaze and others burning in the state to a five-year drought that has turned fields and forests to tinder and killed millions of trees across the state, leaving ready fuel for fires.

The Blue Cut fire is the latest wildfire to erupt in an early fire season that has left eight people dead and destroyed hundreds of homes across the state. Officials fear there may be worse to come once seasonal Santa Ana hot winds begin to blow in September.

As of August 13, nearly 3,900 wildfires had burned more than 45,300 hectares of land in California, an increase of almost 50 per cent over a five-year average.

Of the most destructive fires in California history, nearly all have burned in August, September or October. Four have occurred since 2013 during the last four years of drought.

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