Hundreds of structures have been destroyed by a fast-spreading wildfire in Lake County, California, a fire department spokesman says.

It is understood the fire was threatening more towns in Northern California and continued to send people fleeing, a day after sweeping through the town of Middletown, according to official reports.

California governor Jerry Brown, whose state has been battling several wildfires in recent months, declared a state of emergency in areas hit by the blaze, dubbed the Valley Fire.

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Officials expanded mandatory evacuations as shifting winds sent flames and ash toward a number of towns in the hills north of Napa Valley wine country.

Firefighters conduct a backfire operation to head off the Rocky Fire near Clearlake, California. Photo: Getty

Four firefighters were hospitalised with second-degree burns after they were dropped off by helicopter to battle the fire, which broke out on Saturday in Lake County, north of San Francisco, and exploded in hot weather and drought conditions.

Thousands of evacuees from Middletown, Cobb, Hidden Valley Lake and the Harbin Hot Springs resort gathered in shelters, restaurants and friends’ houses in nearby Kelseyville and Calistoga waiting to hear about their property.

Mark Donpineo, 54, a project manager, said he and two friends were trapped by the fire for four hours on Saturday evening at a golf course in the Hidden Valley Lake gated community.

“There was a little culvert or stream, with a bridge. We stayed there,” he said.

“We got some towels, wetted them down, and basically saw the fire coming.

“You could hear explosions of propane tanks, the ridge was totally on fire, trees were blowing up.”

Schools, homes, businesses destroyed

Local media said a school, blocks of homes, an apartment building and businesses were all destroyed in burned-out Middletown, home to about 1,300 people.

Officials said 1,000 firefighters were working with bulldozers on Sunday to try to contain the flames.

The Rocky fire near Clear Lake, California, has charred more than 27,000 acres. Photo: Getty

“This is one of those fires that has been so fast moving,” Daniel Berlant, public information officer for the California department of forestry and fire protection, Cal Fire, told CBS Sacramento.

The Valley Fire is the latest in a string of wildfires that have ripped through drought-stricken bush and forest on the West Coast.

Firefighters were battling 13 California fires on Sunday including the 1,687 hectare Valley Fire, which was not contained and the 2,639 hectare Butte Fire, which was partly contained.

Chad Greenwood, 44, a travel agent, said he grabbed his two dogs and a suitcase after sheriff’s officers gave him and other residents of an outlying area of Kelseyville an hour to leave.

“Where I live the smoke smell was really bad,” he said.

“It was so dark it looked like it was 8:30 at night, at 7 o’clock in the morning.”

Mr Brown previously declared an emergency in rural Amador and Calaveras counties, where the Butte Fire began on Wednesday and has destroyed 86 homes.

Some 3,500 more people were evacuated on Friday from the path of the flames from a larger blaze, dubbed the Rough Fire, which is burning in Kings Canyon National Park in central California.