Firefighters said they have made good progress battling California’s largest-ever wildfire – but they do not expect to have it fully under control until September.

The blaze north of San Francisco has grown to the size of Los Angeles since it started two weeks ago, fuelled by dry vegetation, high winds and rugged terrain that made it too dangerous for a total of 14,000 firefighters to directly attack the flames.

Crews, including trained prison inmates and firefighters from overseas, have managed to cut lines around half the fire to contain the flames, which now span 470 square miles, the California Department of Forestry and Fire Protection said.

The blaze about 100 miles north of San Francisco around the resort region of Clear Lake has destroyed 116 homes and injured two firefighters.

Those lines have kept the southern edge of the fire from spreading into residential areas on the east side of the lake.

Smoky air
Parts of the state unaffected by the fire itself have suffered poor air quality (AP)

However, Cal Fire said the flames are out of control to the north, roaring into remote and unpopulated areas of thick forests and deep ravines as firefighters contend with record-setting temperatures.

California is seeing earlier, longer and more destructive wildfire seasons because of drought, warmer weather attributed to climate change and home construction deeper into the forests.

Cal Fire battalion chief Jonathan Cox said the area has few natural barriers to slow flames and terrain that firefighters cannot get to.

Wildfires
The fire burns near a residential area near Lake Elsinore (AP)

Instead, firefighters fall back to the nearest road, ridge or river, where they bulldoze a wide line and wait for the flames to come to them.

The Mendocino Complex, which will take months to put out, is one of 18 burning throughout the state.

Because of such extreme conditions early on, officials and experts warn that California could be facing its toughest wildfire season yet, with the historically worst months still to come.

California wildfires
Evacuations have been ordered for several small mountain communities (AP)

No major wildfires are burning near the state capital Sacramento, but for two weeks a dull haze and the faint smell of smoke from distant blazes has blanketed the region.

Campers have been forced to stay inside, as the smoke obscured normally bright skylines and left ash on cars.

Experiencing smoky air from blowing winds is nothing new in many California cities, but Sacramento air quality experts say it is rare for the dirty air to linger for so long, a reality of ever-larger fires that take longer to extinguish.

The haze stretches to the foothills of the Sierra Nevada mountain range, and nearly every major population centre in between has suffered air quality that is considered dangerous for children, the elderly and people with asthma or other respiratory conditions.