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LOS ANGELES — Wildfires that have burned almost 200,000 acres across Southern California showed little sign of abating Thursday night as a new, out-of-control blaze menaced Camp Pendleton and killed at least a dozen racehorses in San Diego County.

The National Weather Service offered no immediate optimism, saying the hot, dry Santa Ana winds fueling the six major fires would bedevil fire crews into late Sunday.

A red flag warning, signaling critical fire weather conditions, was extended through Sunday night for Ventura and Los Angeles counties. The weather service predicted 30-mph winds, with gusts above 60 mph, in open areas and higher elevations, where humidity levels could drop as low as a bone-dry 3 percent.

"This is very, very bad fire weather. We still have evacuation orders that are expanding right now," said Ari Sarsalari, a meteorologist for The Weather Channel.

"It won't be, really, until Sunday that the firefighters can really start to get some of the nitty-gritty work done," he said.

After four days of round-the-clock effort, firefighters were confronted with a new challenge when a brushfire exploded into an out-of-control wildfire about midday Thursday in San Diego County.

Authorities issued mandatory evacuation orders for nearly 20,000 county residents as the so-called Lilac Fire chewed up more than 4,000 acres on its way west toward Camp Pendleton and the city of Oceanside. At least 1,000 homes and other structures were in imminent danger, the California Department of Forestry and Fire Protection said.

Pendleton, the Marine Corps' Western training hub, urged its almost 100,000 personnel and families to remain alert as the evacuation area crept closer to the base and its surrounding community. It said it was sending helicopters and fire crews to help battle the rapidly moving blaze.

Three people suffered burns and were transported to hospitals in San Diego County, authorities said.

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"It looked very eerie, very dark and overcast," Jerry Vasquez, who lives in the area of the Lilac Fire, told NBC San Diego. "It's almost like driving into hell."

Trainers at San Luis Rey Downs, a racehorse training center in Bonsall, said at least a dozen thoroughbreds died when hundreds of horses sprinted from the flames, The San Diego Union-Tribune reported.

Austin Nakatani, whose mother trains a stable at the Downs, told the Daily Racing Form, the national journal of thoroughbred horseracing, that several barns were lost on a property that housed more than 400 horses.

"It's taken 75 percent of San Luis Rey Downs," Nakatani said.

Brian Kozak, a trainer at the facility, told the Union-Tribune: "There's a lot of dead horses. A lot of them just didn't get out of their stalls and got asphyxiated."

The Stronach Group, the largest thoroughbred racing company in North America, called the damage "catastrophic." It said it couldn't confirm how many horses died because access to the facility was restricted.

North of Los Angeles, thick smoke drove air quality readings to "very unhealthy" levels in much of Santa Barbara County. As many as 400 schools across across the San Fernando Valley and coastal areas were closed because of the unhealthy air.

In all, six major fires were burning more than 140,000 acres across a large stretch of Southern California from Santa Barbara County south to San Diego, the California Department of Forestry and Fire Protection said Thursday night. Almost 200,000 residents had been evacuated, and 23,000 homes were threatened, it said.

So far, no deaths had been reported as a result of the fires. But the body of a woman was found near the site of a car accident in Ojai, and investigators were checking whether the blazes figured in her death, Ventura County Sheriff's Sgt. Kevin Donoghue said.

In Ventura County, the Thomas Fire, which first erupted near the town of Santa Paula, completed its westward march to the Pacific Ocean overnight, leaving 115,000 charred acres in its wake. Fire officials said the fire destroyed 427 homes and other buildings and damaged at least 85 others.

"It's definitely moving," Ventura County Sheriff's Capt. Garo Kuredjian told the Los Angeles Times. "Forecasters were correct in terms of the wind forecast for tonight — it's much windier than it was yesterday."

Among those whose homes were destroyed was Tom Lanski, a captain with the Ventura County Fire Department.

"I came off the line and went through our house and found a couple little trinkets," Lanksi said told NBC News' Lester Holt. "I just I grabbed pictures and stuff, because the other things can be replaced. But when you look back in retrospect, I mean, everything was gone so fast."

Lanski, an 18-year veteran, said he'd seen some vicious wildfires in his time, but "I never thought I'd lose my house, ever."

He was allowed to go home to be with his family. But he said he would be back on the lines Friday because "I want to go back out there and help best I can."

California Gov. Jerry Brown, who declared a state of emergency in San Diego, Los Angeles, Ventura and Santa Barbara counties, called on President Donald Trump to declare a federal emergency in Southern California on Thursday night. A federal declaration would direct federal assistance to supplement state and local emergency responses to the fires.

Alex Johnson and Miguel Almaguer reported from Los Angeles. Corky Siemaszko reported from New York.