Californian officials warned Saturday that "extreme" wind conditions were set to fan wildfires across the north of the US state as residents were ordered to evacuate and millions faced power cuts.

Nearly 90,000 people were ordered to flee their homes in Sonoma County, north of San Francisco, a number that rose over the course of the day as the Kincade Fire spread after breaking out on Wednesday.

The blaze, which is burning in remote steep terrain, threatens tens of thousands of structures and had already forced the evacuation of the small community of Geyserville and nearby vineyard operations.

By Saturday evening, a total of 77 structures, among them 31 residential buildings, had been destroyed by the blaze, as more than 2,800 personnel were called to the scene, according to Jonathan Cox, spokesman for the California Department of Forestry and Fire Protection.

"This is definitely an event that we're calling historic and extreme," David King, a meteorologist for the US National Weather Service, told The Los Angeles Times.

"What's making this event really substantial... is the amount of time that these winds are going to remain."

A "strong and potentially historic offshore wind event" was forecast to begin Saturday evening in the San Francisco Bay area, according to the National Weather Service.

The wind was expected to increase overnight and peak Sunday morning, reaching speeds of 60-80 miles per hour (95-130 kilometers per hour), Ryan Walburn, a meteorologist with the National Weather Service San Francisco Bay area, told reporters at a press conference.

Those winds, which have caused a red flag warning indicating a high-risk level, will last into early Monday, meteorologists said.

California's largest utility, Pacific Gas & Electric Co., said it expected to cut off power to 940,000 customers -- a precautionary shutdown that local media reported would affect about two million people.

"The weather event could be the most powerful in California in decades," PG&E said.

"PG&E will need to turn off power for safety several hours before the potentially damaging winds arrive," it added.

"Winds of this magnitude pose a higher risk of damage and sparks on the electric system and rapid wildfire spread."

The Kincade Fire, which is only 10 percent contained, has grown to 25,955 acres (10,500 hectares), Cox said.

By evening, 39,000 people had been ordered to evacuate the area, Mark Essick, sheriff for Sonoma County, told reporters, following a call for 50,000 earlier in the day.

'Don't know what to do'

"I can't explain it," 70-year-old Tina Tavares, who was evacuated from her Geyserville home, told the San Francisco Chronicle.

"It's like you're in a bad earthquake, the ground is opening up... and you're seeing it and don't know what to do."

PG&E has come under fierce scrutiny after power was earlier shut down to nearly 28,000 customers in Sonoma County this week, but some high-voltage transmission lines were still operating when the fire broke out.

The same type of line was responsible for California's deadliest wildfire ever -- last year's Camp Fire, which killed 86 people.

PG&E, which filed for bankruptcy protection earlier this year, has been blamed for several other fires in the state in recent years.

California Governor Gavin Newsom declared a state of emergency and hit out at the company on Friday, saying it had put "profits over the people of California for too long."

He said it was "infuriating beyond words" that a state such as California had to endure blackouts.

"It's about dog-eat-dog capitalism meeting climate change," he said, referring to PG&E. "It's a story about greed, and they need to be held accountable."

Tick Fire turns deadly

Farther south in California, most evacuation orders were lifted after tens of thousands of residents left their homes near Santa Clarita, just north of Los Angeles, due to the so-called Tick Fire that scorched over 4,000 acres.

Officials said Saturday that human remains were found in the burn area, but that authorities had determined the death had not been caused by the fire, according to the Los Angeles Times.

The blaze forced the shutdown of all schools in the area as well as a major freeway, creating traffic chaos for commuters.

Some 1,325 firefighters backed by air tankers and helicopters had battled the flames close to densely packed communities.

Wildfires also erupted over the border in Mexico's Baja California state, where local civil protection authorities said on Friday that three people had been killed and over 150 homes destroyed.

The state's director of civil protection, Antonio Rosquillas, said that the municipality of Tecate, bordering the United States, was the worst hit.