As firefighters battled on the northern front, a second fire swept through the Anaheim Hills, east of Los Angeles, coming within just 24 kilometres of the iconic Disneyland theme park and turning the sky above it a disquieting shade of dark orange.

In northern California there are fires raging in approximately 15 locations spread across eight counties, including Napa, Sonoma and Mendocino, the heart of the state's billion-dollar wine industry.

The deputy director of the California Department of Forestry and Fire Protection, Jane Upton, told US media the fires had already burnt almost 46,000 hectares and consumed up to 1500 buildings, including homes and businesses.

More than 100 people have been treated in area hospitals with fire-related injuries or health issues, including burns and smoke inhalation.

One scene of particular devastation is the city of Santa Rosa, about 80 kilometres north of San Francisco, where whole neighbourhoods have been incinerated.

"I am lucky, my house is fine, my family is fine, my city is not," Santa Rosa's mayor Chris Coursey told media.

The northern California fires have been fanned by unexpectedly fast winds and a lack of humidity, California Governor Jerry Brown said.

"The heat, the lack of humidity and the winds are all driving a very dangerous situation and making it worse," Mr Brown said. "It's not under control by any means."

The fire front east of Los Angeles was sparked only on Monday but spread quickly, fanned by the city's famed Santa Ana winds, which come in October each year.

The fire expanded from about 10 hectares to about 2000 hectares within hours; it has since spread to more than 20 times that area.

The Santa Anas are strong down-slope winds that blow through California's mountain passes towards the coastline; they are warm, dry winds known for severely exacerbating forest fires.

Local news services are reporting that a number of homes are burning and that mandatory evacuation orders have been issued for the Anaheim Hills area.

One media report said seven homes had been destroyed and quoted a local resident saying he had "never seen anything like this" despite living in the area for 21 years.

Airborne ash and smoky conditions forced the closure of at least two major freeways in the vicinity of Anaheim: the State Route 241 and the eastbound side of the State Route 91.

Both fire fronts are unusual in that they are extremely close to populated areas, such as Santa Rose and Glen Ellen, in northern California, and Anaheim, in the east of Los Angeles.

The fire front east of Los Angeles is presently zero per cent contained.

The strong winds made the fire's movement difficult to predict, Anaheim police and fire department spokesman Daron Wyatt told reporters.

"With the wind-driven event, this fire can change behaviour very rapidly," he said.

Despite the proximity of the fire, the Disneyland park is still open; according to reports it is hosting at least two media events this evening, for the Thor: Ragnarok film and the Cars franchise.

At least nine schools in the area have been closed as a precautionary measure.

October is considered a critical month in the Californian calendar as it combines high temperatures with low humidity, leaving the dry inland of the state most vulnerable to fire.

At least four of the five most destructive bushfires in the state's recent history occurred in October, notably the 1991 Oakland Hills "firestorm", which claimed 25 lives and destroyed almost 3000 buildings.