Multi-million-dollar properties, some owned by Hollywood celebrities, were in the path of the blaze, which exploded overnight, fanned by powerful winds and tinder-dry brush.

Firefighters were struggling to get a foothold in the steep canyons that surround the tony California enclave, where towering flames were devouring hillsides, blanketing the air in choking smoke.

Residents spoke of the terror of a fire that erupted late on Monday night.

"We were completely surrounded," one man, who gave only his first name Alec, told broadcaster KTLA.

"It was like 11 pm. We hear people screaming. I walk outside the sky is bright red.

"Within 45 minutes, it's coming down the hillside and then within an hour after that, we're fully surrounded, houses burning down on one side. Ridge is burning on the other side, mountains all around us.

"It got pretty scary at one point."

- Door-to-door -

Los Angeles County Sheriff Robert Luna said around 18,000 people and more than 8,000 properties were in the area threatened by the blaze, which has been dubbed the Franklin Fire.

"The safety of our community members are our number one priority," he told reporters.

Deputies had gone door-to-door urging people to leave the area as the flames began to spread on Monday night.

People described having only moments to flee as embers whipped up by the wind rained down around them.

"I think I'm in shock right now," Malibu City Councilmember Bruce Silverstein told the Los Angeles Times.

Silverstein said he and his wife had packed their bags and fled to a Santa Monica hotel, from where they had watched the progress of the fire on the internet-connected cameras around their property.

"My house has come very close to catching fire multiple times in the last couple of hours" but had been doused by firefighters.

"We thought it was completely under control, then a bunch of embers came flying into the yard."

- Air drops -

By Tuesday morning, the fire had swelled to more than 2,200 acres (900 hectares) and was still raging out of control, with the National Weather Service warning of a high-danger "red flag warning" because of powerful winds.

Firefighters were attacking the blaze from the ground and from the air, with regular drops of red flame retardant around its perimeter in a bid to stem the spread of the flames.

California Governor Gavin Newsom said the state had already secured a Fire Management Assistance Grant from the Federal Emergency Management Agency to assist with the response.

"Fire officials and first responders are working relentlessly to protect lives and property from the Franklin Fire," Newsom said in a statement.

"California is grateful for this federal support, which bolsters these efforts. I urge all residents in affected areas to stay alert and follow evacuation orders."

Wildfires are a feature of life in California and other parts of the US West, and are often exacerbated at this time of year by Santa Ana winds, which blow dry desert air from the interior.

Human-caused climate change is known to increase the likelihood and the severity of fires, with evolving weather patterns leading to longer, hotter dry periods that desiccate vegetation.

Experts say decades of policies stressing a zero-tolerance approach to wildfires have also left much of the countryside overstocked with flammable material, so when blazes erupt they tend to be faster, hotter and more destructive.