Firefighters from several U.S. states arrived in California on Monday after a lightning storm sparked more than 840 wildfires, mostly in the northern part of the state.

Thousands of firefighters battled the blazes on the ground and from the air and Gov. Arnold Schwarzenegger said he was alarmed by the number of fires that kept erupting.

Schwarzenegger said he was told late Sunday evening that the state had 520 fires, and he found it "quite shocking" that by morning the number had risen above 700. That figure was udpated again to 842 fires.

"This is an unprecedented lightning storm in California, that it lasted as long as it did, 5,000 to 6,000 lightning strikes," said Del Walters, assistant regional chief of the California Department of Forestry and Fire Protection.

"We are finding fires all the time."

Schwarzenegger said he had enlisted the help — mostly firefighting aircraft — over the weekend "because you can never prepare for 500 or 700 or 800 fires all at the same time."

Part of the reason for the swelling number of wildfires was that local and state officials were still counting after the fierce thunderstorm Friday night that touched off the blazes.

"We didn't get real lucky with this lighting storm," Walters said. "It wasn't predicted  which often happens with these storms that come in off the Pacific."  

In Mendocino County alone there were 110 fires, with just 17 contained. Two of the biggest fires had each charred nearly 15 square kilometres.

One threatened about 250 homes about 65 kilometres southwest of Sacramento, said Kevin Colburn, a spokesman for the California Department of Forestry and Fire Protection. It was 60 per cent contained Monday.