Two small blazes burning through Northern California have grown at breathtaking speed to form a massive inferno, quickly becoming the largest active bushfire in the state.

It is only a few thousand hectares of charred land away from setting a new mark for destruction.

The twin fires, collectively known as the Mendocino Complex Fire, have together more than doubled in size in just the past four days and burned through 110,750ha of parched land - an area almost the size of Los Angeles.

It has become the second-largest California fire on record, surpassing the size of a massive blaze that killed 15 people in 2003 in San Diego County.

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As fires ravaged the Golden State, US President Donald Trump weighed in, firing off tweets that seem to point fingers, not at the toll of climate change, but at California's environmental laws and use of water resources.

The Mendocino Complex Fire shows little sign of slowing down. Fuelled by low humidity, triple-digit temperatures and winds blowing across wide swaths of tinder-dry vegetation, the conflagration has expanded to three counties, surrounded an entire river and parts of neighbouring reservoirs, and destroyed and damaged nearly 170 homes and other structures.

"There's some challenges that firefighters are facing near the fire and in the area of the fire. We have strong, erratic winds and what that's doing is blowing embers and it's spreading the fire," Captain Thanh Nguyen, a spokesman for a Southern California fire department who's acting as spokesman for the California Department of Forestry and Fire Protection, or Cal Fire, said. "You got steep terrain that makes it difficult for firefighters."

Typically, temperatures dip and humidity rises overnight, giving fire crews a window to slow down the spread of the wildfires. But Nguyen said these have not happened in the affected areas.

The Mendocino Complex Fire began a little more than a week ago with two neighbouring fires burning through only 3845ha of land, then rapidly spreading - at one point, by nearly 12,140ha within hours. The two fires, which have threatened more than 9000 structures, have not merged, but officials are counting them as one.

A 747 Global Airtanker makes a drop in front of advancing flames from a fire in Lakeport, California. Photo / AP
A 747 Global Airtanker makes a drop in front of advancing flames from a fire in Lakeport, California. Photo / AP

The Ranch Fire, the bigger of the two blazes, has continued to grow in multiple directions, threatening communities in its path. The smaller fire, known as River Fire, is more contained.

Firefighters are unlikely to see some respite. Temperatures will slightly dip to the low 90s and high 80s this week, but no rain is in the forecast.

Seventeen fires are burning up and down California. If the Mendocino Complex Fire continues to grow, it could become the largest fire the state has seen in nine decades, surpassing the Thomas Fire, which burned through nearly 114,120ha of land in December 2017 in Ventura and Santa Barbara counties.

Further north in the Redding area, the Carr Fire, which is now 45 per cent contained, has charred more than 165,965ha and destroyed more than 1000 homes.