A wildfire burns about 30 kilometres north of downtown Los Angeles on Monday morning. (Morry Gash/Associated Press)

Two huge wildfires driven by strong Santa Ana winds threatened neighbourhoods near Los Angeles on Monday, burning dozens of mobile homes, forcing frantic evacuations on smoke-choked highways and causing at least two deaths.

A man who appeared to have been a transient living in a makeshift shelter was one of the dead, authorities said. The other was a motorist who was killed about midday Monday in a head-on crash on a freeway entrance ramp as traffic tried to turn around to escape flames.

'It is a blowtorch we can't get in front of.'— Insp. Frank Garrido

Firefighters were struggling with a 2,000-hectare blaze in the San Fernando Valley's northeastern corner when a new blaze erupted at midmorning a few kilometres to the west in mountains above the Porter Ranch area and quickly grew to 800 hectares as wind blew up to 80 kilometres an hour, with gusts reaching more than 100 km/h.

"It is a blowtorch we can't get in front of," said Insp. Frank Garrido of the Los Angeles County Fire Department.

The first fire was burning where neighbourhoods abut rugged canyons below the mountainous Angeles National Forest.

Los Angeles County fire Capt. Mark Savage said as many as 38 mobile homes were destroyed by that blaze early Monday. About 1,000 firefighters from multiple agencies were deployed.

"We could have had an army there and it would not have stopped it," Battalion Chief Mario Rueda said. "Wind is king here. It's dictating everything we are doing."

Winds turn vegetation to tinder

The dry and warm Santa Ana winds typically blow between October and February. As they whistle through Southern California canyons and valleys, they accelerate, drying out vegetation and hastening the spread of any fires that erupt.

"This is what we feared the most," Savage said. "The winds that were expected, they have arrived."

Flames jumped the Foothill Freeway, which was closed in both directions for about a five-kilometre stretch in northern Los Angeles between the 118 Freeway and Interstate 5 amid the morning rush hour, officials said.

"That was quite a jump. That's an eight-lane fire break," said fire spokesman Insp. Paul Hartwell.

The Red Cross said about 500 people have registered at an evacuation centre at San Fernando High School. Agency spokesman Nick Samaniego said some evacuees had seen news footage of their homes burning.

"You can imagine, it's a devastating situation," he said. "A lot of people on pins and needles waiting to hear news about their communities."

Also Monday, a blaze charred more than half of San Francisco Bay's largest island but spared scores of historical structures, including an immigration station that was the first stop for millions of immigrants, mostly from China, in the early 1900s. The Angel Island wildfire was about 75-per-cent contained Monday afternoon; the cause remains under investigation.

With files from the Associated Press