Some of California’s largest-ever fires raged across the US state on Saturday, forcing tens of thousands from their homes as forecasters warned of further blazes sparked by lightning.

Several thousand lightning strikes in recent days ignited fires that left thick smoke blanketing the region.

The total area burned for all the fires in California this week is “close to one million acres [400,000ha]”, CalFire public information officer Jeremy Rahn said.

The agency said on Twitter: “More lightning is anticipated tomorrow and into Tuesday – everyone must remain alert and have an evacuation plan.”

The National Weather Service added: “With severe drought and exceptionally dry fuels present, dry thunderstorms could spark additional wildfires this weekend.

“The western US and Great Plains are shrouded under a vast area of smoke due to ongoing wildfires that extend from the Rockies to the West Coast.”

The two largest blazes – dubbed the LNU Lightning Complex and the SCU Lightning Complex – have burned about 600,000 acres and destroyed 565 structures.

The LNU fire covered 314,207 acres by Saturday morning, making it the second-largest fire in California history. It was 15 per cent contained.

The SCU fire had burned 291,968 acres and was the third-largest fire in state history. It was 10 per cent contained.

Rahm said about 2,600 firefighters are tackling the two blazes, out of 13,700 battling “nearly two dozen major fires”.

California Governor Gavin Newsom said on Friday: “We simply haven’t seen anything like this in many, many years.”

Wineries in the famed Napa and Sonoma regions, which are still reeling from blazes in recent years, are under threat.

Assemblyman Jim Wood of the Healdsburg district in Sonoma told The Los Angeles Times: “Many of these firefighters have been on the lines for 72 hours, and everybody is running on fumes.

“Our first responders are working to the ragged edge of everything they have.”