Crews from across the western US, military planes and National Guard troops have poured into California to join the fight against fires burning across the state, as officials warned of more dry lightning storms approaching.

The worst of the blazes, including the second and third-largest wildfires in recorded California history, were burning in and around the San Francisco Bay Area on Sunday.

More than 200,000 people have been told to flee their homes.

“Extreme fire behaviour with short- and long-range spotting are continuing to challenge firefighting efforts,” the California Department of Forestry and Fire Protection said of the largest conflagration, the LNU Lightning Complex.

The fires, which were ignited by lightning from dry thunderstorms, have killed at least six people and destroyed about 700 homes and other structures.

Nearly 405,000 hectares have been blackened, the department said.

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The LNU Complex, which began as a string of smaller fires that merged into one massive blaze, has burned across about 138,000 hectares.

It was only 17 per cent contained as of Sunday afternoon. To the south, the SCU Lightning Complex was 137,000 hectares and only 10 per cent contained.

Outside the Bay Area, flames threatened forests near the University of California at Santa Cruz and a wide swathe between San Francisco and Sacramento.

Reinforcement crews and fire engines have arrived from Arizona, Montana, Nevada, Texas and Utah, with more on the way. About 200 National Guard members were activated and the US military sent planes, Cal Fire said.

US President Donald Trump declared the fires a major disaster, freeing up federal money to help residents and businesses.

More dry thunderstorms are forecast on Tuesday (local time) as a record heat wave that has baked the state for more than a week showed no sign of ending.

-AAP