Fueled by the return of strong winds, the wildfires burning through California wine country exploded in size and number Wednesday as authorities issued new evacuation orders and the death toll climbed to 21 - a figure that was expected to rise higher still with around 550 people unaccounted for.

Three days after the fires began, firefighters were still unable to gain control of the blazes that had turned entire Northern California neighbourhoods to ash and destroyed at least 3500 homes and businesses.

"We are literally looking at explosive vegetation," said Ken Pimlott, chief of the California Department of Forestry and Fire Protection. "It is very dynamic. These fires are changing by the minute in many areas."

The entire historic town of Calistoga, population 5000, was evacuated. In neighbouring Sonoma County, authorities issued an evacuation advisory for the northern part of the town of Sonoma and the community of Boyes Hot Springs. By the time the advisory was issued, lines of cars were already fleeing.

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"That's very bad," resident Nick Hinman said when a deputy sheriff warned him that the driving winds could shift the wildfires toward the town of Sonoma proper, with 11,000 residents. "It'll go up like a candle."

Ash snowed over the Sonoma Valley, covering windshields, as winds begin picking up toward the potentially disastrous forecast speed of 50kmh. Cars of evacuees raced away from the flames while countless emergency vehicles raced toward them, sirens blaring. Residents manhandled canvas bags into cars jammed with possessions or filled their gas tanks.

Fires burning in northern California at the moment. Photo / Nasa
Fires burning in northern California at the moment. Photo / Nasa

The wildfires ranked as the third deadliest and most destructive in state history. And officials warned the worst was far from over.

"Make no mistake, this is a serious, critical, catastrophic event," Pimlott said. The fires have burned through a staggering 686 square kilometres of urban and rural areas. High winds and low humidity made conditions ideal for fire on the start virtually anywhere on ground that was parched from years of drought.

California Department of Forestry and Fire Protection spokesman Daniel Berlant said 22 wildfires were burning Wednesday, up from 17 the day before. As the fires grow, officials voiced concern that separate fires would merge into even larger infernos.

People salvage remains from Journey's End mobile park in Santa Rosa. Photo / AP
People salvage remains from Journey's End mobile park in Santa Rosa. Photo / AP

"We have had big fires in the past. This is one of the biggest, most serious, and it's not over," Governor Jerry Brown said at a news conference, alongside the state's top emergency officials. They said 8000 firefighters and other personnel were battling the blazes and more resources were pouring in from Oregon, Nevada, Washington and Arizona.

Flames have raced across the wine-growing region and the scenic coastal area of Mendocino farther north, leaving little more than smouldering ashes and eye-stinging smoke in their wake. Whole neighbourhoods were levelled, with only brick chimneys and charred appliances to mark sites that were once family homes.

A wildfire from a distant mountain burns over a vineyard in Kenwood, California on Tuesday. Photo / AP
A wildfire from a distant mountain burns over a vineyard in Kenwood, California on Tuesday. Photo / AP

In Boyes Hot Springs, residents for days had watched the ridges over the west side of town to gauge how close the billowing smoke and orange flames of the wildfires had come. On Wednesday, the ridges themselves were obscured by the growing clouds of smoke.

Increasingly large pieces of gray ash drifted down on the community. Sirens wailed. Residents who had held out hope of staying at home, packed up to leave.

Sonoma County Sheriff Robert Giordano said hundreds of people were still reported missing. But officials believe many of those people will be found. Chaotic evacuations and poor communications over the past few days have made locating friends and family difficult.

A resident flees with a suitcase as fire begins to consume the Overlook Apartment building in Santa Rosa on Monday. Photo / AP
A resident flees with a suitcase as fire begins to consume the Overlook Apartment building in Santa Rosa on Monday. Photo / AP

The sheriff also expects the death toll to climb.

"The devastation is enormous," he said. "We can't even get into most areas."

Pimlott said 73 helicopters, 30 air tankers, 550 fire trucks and nearly 8000 firefighters were being used. Until now the efforts have focused on "life safety" rather than extinguishing the blazes, partly because the flames were shifting with winds and targeting new communities without warning.

Fires were "burning faster than firefighters can run, in some situations," Emergency Operations Director Mark Ghilarducci said.

In Southern California, cooler weather and moist ocean air helped firefighters gain ground against a wildfire that has scorched more than a dozen square miles.

Orange County Fire Authority Capt. Steve Concialdi said the blaze was nearly halfway surrounded and full containment was expected by Saturday, but another round of gusty winds and low humidity levels could arrive late Thursday.

- AP