California wildfires 2020
© AP
Firefighters take refuge in their trucks in a cleared field as a wildfire also known as the Hennessey Fire jumped Knoxville Berryessa Road, west of Sacramento
Lightning-sparked wildfires in California have exploded in size to become some of the largest in state history, forcing thousands to flee and destroying hundreds of homes.

More than 12,000 firefighters assisted by helicopters and air tankers continue to battle wildfires throughout northern California.

Three groups of fires, called complexes, are burning north, east and south of San Francisco and 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 firefighters 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 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 aid.

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

The SCU Lightning Complex fire burning east of San Francisco started on Tuesday and the slightly smaller LNU Lightning Complex burning in wine country which was sparked a day earlier have already become among the 10 largest wildfires in state history.

Assistance from rest of the United States started to arrive on Friday, 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.

Mr Newsom thanked President Donald Trump's administration for its help a day after pushing back on Mr Trump's criticism of the state's wildfire prevention work, saying that he has a "strong personal relationship with the president."

"While he may make statements publicly, the working relationship privately has been a very effective one," 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 of the fires were sparked by thousands of lightning strikes earlier in the week.

Tens of thousands of homes were threatened by flames which spread through dense and bone-dry trees and brush.

Some fires doubled in size within 24 hours, fire officials said.

The death toll has already reached at least six since the majority of blazes started less than a week ago.

Five deaths involved fires burning in wine country north of San Francisco, while the other death was a helicopter pilot who crashed while dropping water on a blaze in Fresno County.

Henry Wofford, spokesman for the Napa County Sheriff's Office, said three of the bodies were found Thursday in a burned home.

The area was under an evacuation order due to "very, very heavy" fire that he said burned multiple homes.

In neighbouring Solano County, Sheriff Thomas Ferrara reported the death of a man, and the other victim was a Pacific Gas and Electric utility worker who was found dead on Wednesday in a vehicle in the Vacaville area.

At least 14,000 people in Solano County remained under mandatory evacuation on Friday, Solano County Undersheriff Brad DeWall said. He said 119 homes have been destroyed in his county.

At least two other people were missing and more than 30 residents and firefighters have been injured, authorities said.

In Napa County, Crosswalk Community Church has transformed its sanctuary and gymnasium into an evacuation shelter, filling the floor with cots spaced at least sux feet apart.

Pastor Peter Shaw said the church has seen a steady stream of people stopping for resources. Some were just looking for information, while others needed gift cards for food and basic needs.

"Covid-19 complicates everything," Mr Shaw said in an email. "Socially distanced cots drastically decreases our capacity."

A few people have stayed the night, he said, adding several people parked their RVs in the church parking lot.

"The longer the evacuations stay in place, I suspect the more people we will see," he said.