LOS ANGELES — Even as crews made significant progress against numerous wildfires across Southern California, fire conditions remained critical, and new blazes continued to drive thousands of residents from their homes Thursday night.

Residents were allowed to return to their homes on Thursday night after the Hillside fire damaged or destroyed two dozen homes and forced the evacuations of at least 1,300 people in San Bernardino County east of Los Angeles. About the same time, a brush fire sparked in Ventura County, northwest of Los Angeles, and exploded to almost 4,000 acres in about three hours, authorities said. By Thursday night, it was around 5,000 acres.

The Maria fire ignited on South Mountain, between the cities of Somis and Santa Paula, about 6:15 p.m. and was rapidly spreading westward, the Ventura County Fire Department said. County officials issued mandatory evacuation orders for about 7,500 residents and said only that the cause was under investigation.

No structures have been destroyed, and more than 400 firefighters were fighting the fire and protecting homes and other structures as well as orchards, fire officials said.

"It started, just a very small corner and then like within minutes it just expanded really quickly,” Davawn Escobar told NBC Los Angeles with flames visible on a hillside. "Really quickly. But the firemen were on top of it because within like five minutes you just saw a huge line of them just driving up the hill.”

"We just had the Thomas fire two years ago — and that ... was horrible in itself," referring to the 2017 blaze that destroyed more than 1,000 structures in Ventura and Santa Barbara counties after it broke out on Dec. 4 near Santa Paula. Two were killed in the fire, and heavy rain in the burn scar unleashed a debris flow in the coastal community of Montecito killed more than 20 people.

Ventura County Fire Assistant Chief John McNeil sought to reassure the public that the Maria Fire is not expected to be a repeat of that massive and destructive fire two years ago.

"I want to assure you that this is not the Thomas Fire," he said. "Based on the location, it's going to eventually run out of fuel ... we’re looking at maybe 12,000 acres at the biggest footprint on this," he said.

Conditions remained dangerous in Ventura County on Thursday, with sustained winds approaching 25 mph in a bone-dry area where relative humidity was stuck in the single digits.

The danger would have been even worse, but strong Santa Ana winds gusting as high as 70 mph had ebbed on Thursday, allowing firefighters to get ahead of dozens of fires that ignited earlier in the week.

Ventura County Fire Chief Mark Lorenzen said that while the Maria Fire appears very large and close at night, it is north of the 118 freeway "and it is our intention to keep it there." In addition to the weather which fueled the blaze, someone flew a drone in the fire zone and that hampered aerial firefighting efforts, Sheriff Bill Ayub said.

Firefighters managed to hold the Hillside fire, in San Bernardino County about 70 miles east of downtown Los Angeles, to about 200 acres at 50 percent containment, fire officials said Thursday night. Six homes were destroyed, 18 others were damaged and a firefighter was overcome by smoke from the flames, which were reported about 1:40 a.m.

Authorities ordered mandatory evacuations of about 490 homes with about 1,300 residents. The orders were lifted Thursday evening for residents only, authorities said.

The cause of the fire wasn't immediately known, but investigators said no overhead power lines were near the area where the blaze ignited.

Susan Mercado said she was grateful that firefighters got to her San Bernardino County house in the nick of time.

"We took the kitties, ourselves and our cellphone, and that's it," Mercado told NBC Los Angeles. "The fire was coming through our yard. It was life or death."

She added: "I don't know how they saved our house."

In the nearby Riverside County community of Jurupa Valley, firefighters were also able to bring the 46 fire to 50 percent containment by Thursday night, officials said.

The blaze, which started Wednesday, had consumed at least 300 acres and forced the evacuations of two mobile home parks and a psychiatric nursing care facility, officials said.

Elderly people wearing masks and wrapped in blankets were taken out in wheelchairs and on gurneys as smoke swirled overhead.

"There was one moment when I could see nothing but dark smoke, and I was like, 'We're going to die,'" Qiana McCracken, assistant director of nursing for the Riverside Heights Healthcare Center, told NBC Los Angeles.

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Kyla Brown, assistant director of the Riverside County Regional Park and Open Space District, said the fire destroyed the 40-acre Louis Robidoux Nature Center in Jurupa Valley "despite the heroic work of CalFire and all the first responders."

No animals were at the park, which has been closed since 2017.

But the county Animal Services Department said that numerous animals were threatened at other properties and that so far, four dogs, a cat, 15 goats, two horses and 15 rabbits had been rescued.

All appeared uninjured, it said.

These are the goats we evacuated early this morning in response to the #46Fire. pic.twitter.com/4HjeFcIdVi — RivCO animalSERVICES (@helpinRIVcoPETS) October 31, 2019

The Getty fire in West Los Angeles, which started Monday and had burned at least 750 acres, was up to 52 percent containment by Thursday night. At least 12 homes had burned and five others had been damaged, officials said.

Meanwhile, the 1,806-acre Easy fire in Simi Valley reached 60 percent containment by Thursday night, state fire authorities said. A mandatory evacuation order was also lifted on Thursday.

More than 800 firefighters were still in action Thursday.

Gesturing toward a field of brush, some of it apparently still smoking, Mike DesForges, a spokesman for the Ventura County Fire Department, warned that it could re-ignite at any moment.

That blaze, which broke out Wednesday, initially threatened the Ronald Reagan Presidential Library. The library was closed again on Thursday.

Large parts of California have been under rolling power blackouts to prevent downed or damaged power lines from sparking new fires.

Pacific Gas and Electric Corp., the largest electric utility in the state, said only about 6,000 homes and businesses, roughly equivalent to about 15,000 people, remained without power in the northern part of the state Thursday night. PG&E said it had discovered 143 "incidents of damage or other hazards" that could have led to new fires had the power not been shut off — and that number was expected to rise.

At one point this week, as many as 2 million Northern Californians were without power as the third and fourth major fire-safety shutoffs of the month overlapped, re-energizing widespread criticism of the utility, whose performance Gov. Gavin Newsom has called "unacceptable."

Marshall Hospital in Placerville, in El Dorado County, used four generators to keep some of the lights on, closed some clinics and rescheduled scores of elective surgery procedures.

"Several hundred patients have been impacted, if not more," Cindy Rice, the Hospital's vice president of clinical nursing, told NBC affiliate KCRA of Sacramento.

Alex Johnson reported from Los Angeles, and David K. Li from New York.