A series of historically deadly Northern California wildfires have flared up again in strong winds.

Blazes have roared through parched hills and vineyards, prompting more evacuations from an arc of flames that has killed at least 21 people, destroyed more than 2000 buildings and battered the region's renowned wine-growing industry.

Officials ordered more evacuations in flame-battered Sonoma County, where one of the blazes, known as the Tubbs Fire, has already killed 11 people.

Fire from a distant mountain burns over a small fire in Kenwood, Calif. Photo / AP
Fire from a distant mountain burns over a small fire in Kenwood, Calif. Photo / AP

The massive fire, one of several that has been ravaging the region since Sunday, advanced overnight towards populated areas, prompting deputies to "run towards the fire, banging on doors, getting people out of their houses," said Misti Harris, a Sonoma County Sheriff's Office spokeswoman.

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"It's rapidly changing, it's moving quickly, it's a very fluid situation. The fire is growing," she said.

Tubbs is California's deadliest wildfire since 2003, when 15 people were killed in the Cedar Fire in San Diego County. Hundreds of people are missing in Sonoma County, and authorities expect the death toll to rise.

The sheriff's office said authorities have found more than 100 people who were reported missing, but that about 550 still remain unaccounted for.

It's unclear if those who are missing have been harmed, or are simply unable to reach friends and families, as fires have disabled much of the communication system in the region.

Firefighter Nick Gonzalez-Pomo, of the San Rafael Fire Department, waters down hot spots on a garage. Photo / AP
Firefighter Nick Gonzalez-Pomo, of the San Rafael Fire Department, waters down hot spots on a garage. Photo / AP

"This has been one of the deadliest weeks for fires that we've experienced in recent time," Daniel Berlant, assistant deputy director for Cal Fire, said. "And a lot of that has to do with the fact that the fires ignited overnight. Many people were asleep when the fire started. Getting them evacuated was an extreme challenge for rescue crews."

The two biggest wine-country fires, Tubbs and one known as Atlas that began in Napa County, grew overnight as conditions worsened and had torched a combined 28,300ha by Wednesday morning, according to Cal Fire.

The fast-moving flames have swept through densely populated neighbourhoods over the past two days, causing residents to flee from homes in the middle of the night. One couple had to jump into their pool as flames rushed across their land, taking occasional gasps for air as flames lapped at their backs.

This aerial image shows homes that were destroyed by a wildfire in Santa Rosa, California. Photo / AP
This aerial image shows homes that were destroyed by a wildfire in Santa Rosa, California. Photo / AP

High winds that whipped up 17 large fires had faded earlier Tuesday (local time) and humidity increased, assisting an operation that has drawn resources from throughout the state and neighbouring Nevada. But officials warned that the sharp northern wind, known as a Diablo, would return, allowing only a brief window for firefighters to carve clearings in place to stop the fires from spreading to vulnerable populated areas.

That wind has returned, along with lower humidity levels.

The National Weather Service expects "red-flag" conditions - including wind gusts up to 64km/h - to remain until Thursday in the North Bay Area, which includes Sonoma and Napa counties.

Higher winds could hamper efforts to contain the fires over the next few days, officials said.

On Wednesday morning, as weary firefighters tried to control the fires on the front lines, dozens of fire crews from cities as far away as Bakersfield, more than 480km to the south, were briefed on the deteriorating conditions at a command center set up at the Sonoma County Fairgrounds.

A car lies upside down at the site of a home destroyed by fires in Santa Rosa, California. Photo / AP
A car lies upside down at the site of a home destroyed by fires in Santa Rosa, California. Photo / AP

More than 25,000 people have fled homes from seven counties north of San Francisco, filling dozens of shelters that state officials had hoped to consolidate in the coming days to provide more efficient services. Many left houses with nothing, and officials acknowledged it could be weeks before some are able to return. In Sonoma County, 5000 people have taken refuge in 36 shelters as of Wednesday morning, officials said.

"These fires came down into neighbourhoods before anyone knew there was a fire in many cases," Cal Fire Chief Ken Pimlott said. "This is just pure devastation and it's going to take us a while to get out and comb through all of this."

The scope of the damage prompted President Trump to approve federal emergency assistance to California, agreeing to a request made by Governor Jerry Brown. The declaration, announced by Vice President Pence during a visit to the state's Office of Emergency Services near Sacramento, provides immediate funds for debris clearing and supplies for evacuation centres, among other aid.

Leslie Garnica searches for belongings in the ashes of her home that was destroyed by fire in the Coffey Park area of Santa Rosa, California. Photo / AP
Leslie Garnica searches for belongings in the ashes of her home that was destroyed by fire in the Coffey Park area of Santa Rosa, California. Photo / AP

"I appreciate the fast response from the President," Brown said.

The fires are the most destructive in what already has been a severe wildfire season for California and much of the West, where more than 3 million ha have been charred this year. In his letter to Trump, Brown said nearly 7500 fires have flared in California this year. Ten of them have prompted him to declare a state of emergency.

As a thick haze coated the sky and settled into the region's canyons and valleys, state officials remained focused on rescue and containment.

The cause of the fires, which flared overnight and blew swiftly through more than 7000ha in the following days, was unknown and likely to remain so for some time.

Pimlott said the possibility that a lightning strike started the fires was "minimal". In California, he said, 95 per cent of wildfires are started by people, inadvertently or intentionally. "All of these fires remain under investigation," he said.

Flames fed by a gas line, burns in the debris of a fire ravaged home. Photo / AP
Flames fed by a gas line, burns in the debris of a fire ravaged home. Photo / AP

State officials said firefighters planned to clear lines between the Atlas Fire and the city of Napa, and between the Tubbs Fire and the city of Santa Rosa - the largest in Sonoma County and gateway to the wine-tourism industry.

Those barriers would protect the areas from the south where winds were expected to shift back to the north in the days ahead.

Officials said the idea, in the case of the Tubbs Fire, was to prevent a "reburn" of Santa Rosa.

For Dylan Sayge, the original burn was devastating. He and his roommates were awake early Monday morning when they noticed an unusual sight outside their US$1600-a-month rental home in the Coffey Park neighbourhood of Santa Rosa.

"We realised ash was falling from the sky," said Sayge, 23, a musician who works at Trader Joe's.

This aerial image shows a neighborhood that was destroyed by a wildfire in Santa Rosa. Photo / AP
This aerial image shows a neighborhood that was destroyed by a wildfire in Santa Rosa. Photo / AP

Soon after, online, they learned that a fast-moving fire had jumped Highway 101, propelled by howling winds. The power flickered and an explosion followed as a transformer blew nearby. They grabbed their dogs - Cash, Willie and Shorty - and their cat, Apollo. Sayge packed up baby pictures and musical instruments.

They headed out in three cars and into a traffic jam. Sayge left behind a 1998 Ford Taurus that he had just been gifted. The dense smoke clouded visibility. He eventually made it to a friend's home in Fairfax, in Marin County.

The next day, he learned that the house was gone, the Taurus a charred husk.

"The world can change in any moment," Sayge said. "Anytime."