Flames raced down a steep Southern California hillside "like a freight train," forcing crews to clear as many as 10,000 homes by Monday as a wildfire raged through tinder-dry canyons there.

The fire has destroyed at least 18 homes in northern Los Angeles County. It gained ferocious new power two days after it broke out, sending so much smoke in the air that planes making drops on it had to be grounded for part of the day Sunday.

Fire officials said they've ordered about 20,000 residents to leave their homes.

"For this time of year, it's the most extreme fire behavior I've seen in my 32-year career," County fire Chief Daryl Osby said.

Officials said late Sunday that the blaze had burned through at least 51 square miles of brush north of Los Angeles -- but that number is expected to jump Monday when better assessment is done at daylight. It's just 10 percent contained.

Helicopters made water drops through the night. New evacuations were put in place Sunday as the blaze roared to the northeast through Angeles National Forest toward the city of Acton.

About 300 miles up the coast, crews were battling another fire spanning more than 17 square miles that destroyed six homes on Sunday and forced evacuations outside the scenic Big Sur region.

The Southern California blaze has blackened at least 46 square miles of brush on ridgelines near the city of Santa Clarita. Osby said the size estimate could grow considerably once better assessment is done.

Planes were unable to make drops over the fire for a long stretch of the afternoon before resuming for a few hours before dusk. Helicopters released retardant around the perimeter of the fire all day and would continue into the night.

"The fire's just doing what it wants right now," U.S. Forest Service spokesman Nathan Judy said. "We have to stick back, let it do what it wants to and attack it where we can."

Juliet Kinikin said Sunday there was panic as the sky became dark with smoke and flames moved closer to her home a day earlier in the Sand Canyon area of Los Angeles County.

"And then we just focused on what really mattered in the house," she told The Associated Press.

Kinikin grabbed important documents and fled with her husband, two children, two dogs and three birds. They were back at home Sunday, "breathing a big sigh of relief," she said.

Residents of thousands of homes were evacuated as shifting winds were pushing flames northeast through Angeles National Forest, authorities said.

Lois Wash, 87, said she and her daughter and her dog evacuated, but her husband refused.

"My husband's stubborn as a mule, and he wouldn't leave," Wash told KABC-TV. "I don't know if he got out of there or not. There's no way of knowing. I think the last time I looked it was about 100 yards from us. I don't know if our house is still standing or not. All we can do is pray."

The fire has ripped through brush withered by days of 100-degree temperatures and years of drought.

"It started consuming houses that were non-defendable," Los Angeles County Deputy Fire Chief John Tripp said, describing the flames as charging through terrain "like a freight train."

More than 1,600 firefighters were battling the flames that sent up a huge plume of smoke visible across the region.

The body of a man was discovered Saturday in a burned sedan outside a home in the fire zone. Los Angeles County sheriff's officials are investigating the death.

The fire destroyed film sets at Sable Ranch in Santa Clarita, which has Old West-style buildings used for movie locations. It also forced a nonprofit sanctuary for rescued exotic creatures to evacuate 340 of its more than 400 animals, including Bengal tigers and a mountain lion.

North on the Central Coast, a blaze consuming brush in rugged mountains near Big Sur was threatening about 1,650 homes. It burned in inaccessible terrain 5 miles south of Garrapata State Park and forced the communities of Palo Colorado and Carmel Highlands to evacuate, California's forestry department said.

Brock Bradford lives in a historic house in Palo Colorado and could see the flames coming down the road as he evacuated.

"I hope I don't have to rebuild my house," he told the Monterey Herald. "I'm 66."

The Associated Press contributed to this report.