Lightning-sparked wildfires in Northern California spread on Friday to become some of the largest in the state’s history.

The rapid spread of the blazes forced thousands of people to flee their homes and destroyed hundreds of dwellings and other structures, as reinforcements began arriving to help weary firefighters.

More than 12,000 firefighters aided by helicopters and air tankers are battling fires throughout California.

Three groups of fires, called complexes, burning north, east and south of San Francisco have together scorched 991 square miles, destroyed more than 500 structures and killed five people.

At least 100,000 people are under evacuation orders.

The number of personnel assigned to the sprawling LNU Complex — a cluster of blazes burning in the heart of wine country north of San Francisco — doubled to more than 1,000 firefighters on Friday, Cal Fire Division Chief Ben Nicholls said.

“I’m happy to say there are resources all around the fire today. We have engines on all four sides of it working hand-in-hand with the bulldozers to start containing this fire, putting it to bed,” Mr Nicholls said.

Fire crews with help from “copious amounts of fixed-wing aircraft” were working on Friday to stop a large blaze from reaching communities in the West Dry Creek Valley of Sonoma County, he said.

The blazes, coming during a heat wave that has brought temperatures of more than 37C, are taxing the state’s firefighting capacity.

But assistance from throughout the country has begun to arrive, with 10 states sending fire crews, engines and aircraft to help, Governor Gavin Newsom said.

“We have more people but it’s not enough. We have more air support but it’s still not enough and that’s why we need support from our federal partners,” Mr Newsom said.

There are 560 fires burning in the state, many small and remote but there are about two dozen major fires, mainly in Northern California. Many blazes were sparked by thousands of lightning strikes earlier in the week.

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