Around 14,000 firefighters are battling 18 fires that have carved their way through forest land and are threatening urban areas in California.

“For whatever reason, fires are burning much more intensely, much more quickly than they were before,” said Mark A Hartwig, president of the California Fire Chiefs Association.

Some of the largest fires have erupted within the past few weeks as the state has seen record breaking temperatures — and the historically worst months of wildfire season are still to come.

In northern California, the record breaking Mendocino Complex — twin fires being fought as a single conflagration — gained ground on Tuesday but more slowly because its own smoke covered the area and lowered the temperature, according to the California Department of Forestry.

California Wildfires
Some 14,000 firefighters are tackling the blazes (Noah Berger/AP)

The flames, which had burned 457 square miles, were raging in mostly remote areas and no deaths or serious injuries were reported but 75 homes were destroyed.

The blaze, which broke out on July 27, initially spread quickly because of what officials said was a perfect combination of weather, rugged topography and abundant brush and timber turned to tinder by years of drought.

Resources were also thin at first because thousands of firefighters were already battling a fire hundreds of miles north. That fire, which spread into the city of Redding, killed six people and destroyed more than 1,000 homes. The so-called Carr Fire was less than half contained.

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

In becoming the biggest fire in California’s history, the Mendocino Complex fire broke a record set just eight months ago. A blaze in Southern California in December killed two people, burned 440 square miles and destroyed more than 1,000 buildings.

California’s firefighting costs have more than tripled from 242 million US dollars in the 2013 fiscal year to 773 million dollars in the 2018 fiscal year that ended June 30, according to Cal Fire.

Governor Jerry Brown said: “We’re in uncharted territory. Since civilisation emerged 10,000 years ago, we haven’t had this kind of heat condition, and it’s going to continue getting worse. That’s the way it is.”