A wildfire in Southern California raged virtually unchecked in thick brush on Wednesday after destroying dozens of houses and forcing the evacuation of more than 80,000 people from their homes, officials said.

The so-called Blue Cut Fire ignited on Tuesday in the mountainous Cajon Pass, near a highway corridor between Los Angeles and Las Vegas, and has since covered more than 100sq km.

Firefighters had carved containment lines around only 4 percent of the blaze by Wednesday night, according to fire service officials.

They described the blaze as unusually fierce, even in a year of intense wildfires in the west, where years of drought have put a heavy burden on firefighting resources.

"In my 40 years of fighting fire, I've never seen fire behaviour so extreme as it was yesterday," Michael Wakoski, the incident commander, told a news conference on Wednesday.

As many as eight wildland fires were burning in California on Wednesday, three of them scorching thousands of hectares as firefighters sought help from emergency services in other states and the California National Guard.

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US government forecasters have said that Southern California faces a potential threat from major wildfires until December, given the dryness and warm weather.

The Santa Ana winds, which sweep desert air to California's coast while driving the fires, are due to kick up next month, California Department of Forestry and Fire Protection (Cal-Fire) spokeswoman Lynne Tolmachoff told the Reuters news agency.

September and October are often the peak of the state's wildfire season.

The Blue Cut Fire, named for a narrow gorge north of San Bernardino where it started, threatened the town of Wrightwood near a ski resort and other communities in a partly rural area, authorities said.

The cause of the fire is being investigated.

The Blue Cut Fire has covered nearly 100 square kilometres in only two days [Reuters]

Authorities said they were forced to close part of Interstate 15 on Wednesday and to order about 82,000 residents to evacuate their homes.

Thick columns of smoke blocked out the sky above mountain peaks as the fire overran a number of properties, leaving behind barren lots with blackened appliances and vehicles stripped of nearly everything but metal.

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

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Despite dire warnings from authorities, hundreds of residents have refused to leave their homes, the Los Angeles Times reported. 

Among them was Nico Santucci, who was standing sentry outside his home in Lytle Creek with his young son by his side.

"If the first house catches on fire here or the fire breaks this mountain here, then I'm gone, then we're gone," Santucci said.

About 970km to the northwest, the so-called Clayton Fire was 40 percent contained after charring 1,600 hectares in and around the community of Lower Lake and destroying 175 homes and businesses.

California had spent $164m by August 12 to combat wildfires this year, Cal-Fire's Tolmachoff said, not including the Blue Cut or Clayton fires.

Source: Agencies