California's biggest wildfire on record is expected to burn for the rest of the month, fire officials say, amid strong winds and a summer heatwave.

Hot and blustery conditions challenged thousands of fire crews, who battled eight major blazes burning out of control across the state on often rugged terrain.

The inferno has spread more than 450 square miles in just 11 days, forcing thousands of people to flee and destroying at least 75 homes. No deaths or serious injuries have been reported.

The latest report from firefighters is that 34 per cent of the fire has been contained, after it damaged 75 residences and nearly 2,000 other structures.

The state experienced a similarly large wildfire last year in the southern part of the state in Santa Barbara and Ventura counties, but the Mendocino Fire has eclipsed the Thomas Fire in size.

The Thomas Fire claimed two lives and destroyed 1,000 buildings in the region.

Apocalyptic scenes as Californian wildfire rages leaving 6 dead

Though the current blaze is raging in mostly uninhabited land, it does threaten more than 11,000 structures surrounding it and as it spreads more evacuations have been ordered.

The state department of forestry and fire protection (Cal Fire) spokesperson Scott McLean said nearly 14,000 firefighters are combatting the Mendocino blaze.

"I can remember a couple of years ago when we saw 10 to 12,000 firefighters in the states of California, Oregon and Washington and never the 14,000 we see now," he said.

Cal Fire chief Charlie Blankenheim said planes have been deployed to drop water across wide swathes of the blaze to assist ground crews as well.

The Mendocino fire has caused such a haze of smoke that Sacremento county, where the state capital is, has advised residents to restrict outdoor activities over health concerns from breathing in the smoke.

Crews also gained ground against another northern California wildfire that has destroyed more than 1,000 homes in and around the city of Redding.

It was nearly halfway contained, Cal Fire said. That wildfire is about 225 miles (360 km) north of San Francisco and started more than two weeks ago by sparks from the steel wheel of a trailer's flat tyre.

The fire has claimed the lives of two firefighters and four residents while displacing more than 38,000 people.

Meanwhile, a new fire erupted south of Los Angeles in Orange County once again in the southern part of the state on early in the week and quickly spread through the Cleveland National Forest.

Campgrounds and homes in the area were ordered evacuated as the fire sent up an enormous pillar of smoke and ash due to dried out earth.

Environmental experts have said one of the causes of the fires spreading so quickly and intensely this year is climate change, which has led to hotter weather drying out vegetation to the point that the smallest spark can cause massive blazes.