The unidentified firefighter was killed on Monday while battling the Ranch Fire, one of two blazes that make up the Mendocino Complex, which has already charred about 141,200 hectares, the California Department of Forestry and Fire Protection (Cal Fire) said.

The Utah firefighter was flown to a hospital where he died, the Los Angeles Times reported.

"Fact finding on the accident is ongoing and notification of the next of kin is in progress," Cal Fire said.

The firefighter was the sixth person killed battling California's intense wildfires this year, which have been some of the most destructive in more than a decade as they have forced tens of thousands to evacuate.

The Mendocino Complex, which has destroyed 146 homes since it began on July 27, has been stoked by persistent hot, dry and windy conditions. Crews have been able to cut containment lines around 68 per cent of the northern California fire, Cal Fire said.

The complex is one of about 110 major wildfires burning across the western United States which have burned more than 23,000 sq km, an area larger than the state of New Jersey, according to the National Interagency Fire Centre.

Though temperatures had dropped from their highs in recent days, they were expected to stay above 32C through Friday. The cooler temperatures gave firefighters on Monday a chance to attack the string of major wildfires across California, fire officials said.

Another massive blaze, the Carr Fire, has blackened about 207,000 acres and killed eight people in and around Shasta County, north of Sacramento near the Oregon state line. It was 63 per cent contained as of Monday afternoon, Cal Fire said.

In Southern California the Holy Fire, which authorities say was set on August 6 by a disgruntled homeowner in an Orange County canyon, was 59 per cent contained after torching more than 22,000 acres and destroying a dozen cabins.

Australian Associated Press