The wildfire started on Tuesday in a part of the state left bone dry by five years of drought, and within a day it had spread across nearly 47 square miles.

There were 1,300 firefighters tackling it, focusing their efforts on the mountain communities of Lytle Creek, Wrightwood and Phelan.

More than 34,000 homes and some 82,000 people were under evacuation warnings, and the authorities implored residents not to think twice if told to leave, although it appears many were staying.

"From reports that we were hearing, possibly up to half didn't leave," said Lyn Sieliet, a US Forest Service spokeswoman.

"It does change the way that we can fight fire," she added. "Now we have to worry about the people in there as well as trying to protect the structures and trying to build a line of defence as the fire comes toward that area."

Six firefighters were briefly trapped by flames, when occupants of a home refused to leave and the crew stayed to protect them.

"This moved so fast," said Darren Dalton, 51, who along with his wife and son had to get out of his house in Wrightwood.

"It went from 'have you heard there's a fire?' to 'mandatory evacuation' before you could take it all in. This is a tight little community up here. Always in rally mode. Suddenly it's a ghost town."

Hundreds of cars packed with belongings and animals left the town. The air for miles around the blaze was filled with smoke.

The authorities could not immediately say how many homes had been destroyed, but they warned that the number will be large.

"There will be a lot of families that come home to nothing," San Bernardino County Fire Chief Mark Hartwig said after flying over a fire scene he described as "devastating".

He added: "It hit hard. It hit fast. It hit with an intensity that we hadn't seen before."

No deaths were reported, but cadaver dogs were searching the ruins for anyone who was overrun by the flames.

The cause of the fire was not immediately known.

The drought has turned the state's wildlands into a tinder box, with eight blazes currently burning from Shasta County in the far north to Camp Pendleton just north of San Diego, and 10,000 firefighters tackling them.

April Christy, who was at an evacuation centre in Fontana, said she had never seen a wildfire like it.

She said: "No joke, we were literally being chased by the fire. You've got flames on the side of you. You've got flames behind you."

One major fire, north of San Francisco, was fading and about 4,000 people in the town of Clearlake were allowed to return home.

Their relief was tempered by anger at a 40-year-old man authorities believe set the blaze that wiped out parts of the small town of Lower Lake over the weekend. That fire destroyed 175 homes and other structures in the working-class community.

Damin Pashilk is charged with 14 counts of arson in connection with 12 separate fires dating back to July 2015 and one count of attempted arson. He appeared in court on Wednesday but did not enter a plea.