A wildfire with a ferocity never seen before by veteran California firefighters raced up and down canyon hillsides, instantly engulfing homes and forcing thousands of people to flee, some running for their lives just ahead of the flames.

By Wednesday, a day after it ignited in brush left tinder-dry by years of drought, the blaze had spread across nearly 47 square miles and was raging out of control. The flames advanced despite the efforts of 1,300 firefighters.

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."

"It hit hard. It hit fast. It hit with an intensity that we hadn't seen before," he said.

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

In 40 years of fighting fires, Incident Commander Mike Wakoski said, he had never seen conditions as extreme as those in Cajon Pass, where the fire broke out Tuesday.

Residents like Vi Delgado and her daughter April Christy, who had been through a major brushfire years before, said they had never seen anything like it either.

"No joke, we were literally being chased by the fire," a tearful April Christy said in a voice choked with emotion as she and her mother sat in their minivan in an evacuation center parking lot in Fontana. They did not go inside because their dogs, three Chihuahuas and a mixed-breed mutt, were not allowed.

"You've got flames on the side of you. You've got flames behind you," Christy said, describing a harrowing race down a mountain road. She was led by a sheriff's patrol car in front while a California Highway Patrol vehicle trailed behind and a truck filled with firefighters battled flames alongside her.

She and her mother, onsite caretakers at the Angels and Paws animal rescue shelter in Devore Heights, said it was only moments after they smelled smoke that flames exploded all around them. They grabbed their pets and tried to rescue nine other shelter dogs and three cats, but a sheriff's deputy told them there was no time.

"You won't make it. Save yourself. Take your truck and leave," Delgado said the deputy shouted at her, adding that he and others would try to rescue the animals. She learned later that authorities did save the animals, but officials could not tell her if her home survived.

More than 34,000 homes and some 82,000 people were under evacuation warnings as firefighters concentrated their efforts on saving homes in the mountain communities of Lytle Creek, Wrightwood and Phelan. They implored residents not to think twice if told to leave, but it appears many were staying.

"From reports that we were hearing, possibly up to half didn't leave," said Lyn Sieliet, a U.S. 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 defense as the fire comes toward that area."

Six firefighters were briefly trapped by flames during the fire's early hours, 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.

Although there was no official count on how many homes were lost, Eric Sherwin of the San Bernardino County Fire Department said Tuesday that he had seen at least a dozen buildings go up in flames, some of them homes. Among them was the Summit Inn, a historic Route 66 diner near Interstate 15.

The interstate is a major route connecting Southern California to Las Vegas, and countless big rigs were parked along it on both sides of Cajon Pass on Wednesday, waiting for it to reopen.

Less than 24 hours after the blaze began 60 miles east of Los Angeles, authorities had assembled a fleet of 10 air tankers, 15 helicopters and an army of 1,300 firefighters, many of them just off the lines of a wildfire that burned for 10 days just to the east.

At a dawn briefing, half the firefighters raised their hands when asked how many had just come from an earlier blaze, part of a siege of infernos burning across California this year. In all, 10,000 firefighters are fighting eight blazes around the state, from Shasta County in the far north to Camp Pendleton north of San Diego.

Meanwhile, a major blaze 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 with anger at a 40-year-old man authorities believe set the blaze that wiped out several blocks 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, who is charged with setting that fire and 12 others dating back to July 2015, appeared in court Wednesday but did not enter a plea.

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Associated Press writers John Antczak, John Rogers, Robert Jablon and Andrew Dalton in Los Angeles, Sudhin Thanawala in Lakeport, Kristin J. Bender in San Francisco and Don Thompson in Sacramento contributed to this report.