Multimillion-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 enclave just outside Los Angeles, where towering flames were devouring hillsides, blanketing the air in choking smoke.

"We were completely surrounded," one man, who gave only his first name, Alec, told broadcaster KTLA about the fire that broke out late Monday night.

"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 response -

Around 2,000 buildings in the eastern half of Malibu were under a compulsory evacuation order. People living in another 6,000 properties in the city were being advised to leave

"The safety of our community members are our number one priority," Los Angeles County Sheriff Robert Luna told reporters.

Deputies had gone door-to-door urging people to get out 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, he said.

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

Actor Dick Van Dyke, who celebrates his 99th birthday on Friday, posted on Facebook that he and his wife had fled their Malibu home.

"Arlene and I have safely evacuated with our animals except for (a cat that) escaped as we were leaving. We're praying he'll be OK and that our community in Serra Retreat will survive these terrible fires," he wrote.

- Airdrops -

By Tuesday afternoon, the fire had swelled to more than 2,600 acres (1,050 hectares) and was still raging out of control, with the National Weather Service posting a high-danger "red flag warning" because of powerful winds.

More than 700 firefighters were battling the blaze, aided by a fleet of water- and retardant-dropping aircraft, including Super Scoopers and tankers capable of dumping 1,200 gallons (4,500 liters) of retardant at a time.

Tens of thousands of people across southern California had their electricity cut off. Utilities frequently de-energize lines ahead of windy days to reduce the fire risk from downed power lines.

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.