A MASSIVE, out-of-control Northern California wildfire that has destroyed 68 homes and forced thousands to flee has become the fifth largest in the region’s history.

And it’s only going to get worse as crews battle high temperatures and strong winds.

The Mendocino Complex Fire, made up of two separate blazes that merged near Ukiah, north of Sacramento, exploded by 25 per cent overnight and had blackened around 1000 square kilometres, the California Department of Forestry and Fire Protection said.

“The Mendocino Complex Fire has charred more than 102,790ha, making it the fifth largest blaze in California’s history,” AccuWeather said on its website. “Firefighters will continue to face local gusty winds and building heat this week.”

The Mendocino is just one of 17 major wildfires burning across California that prompted US President Donald Trump to declare a “major disaster” in the state, ordering federal funding to be made available to help recovery efforts.

PG & E Corporation confirmed on Sunday that 21-year-old apprentice lineman, Jay Ayeta, died when his vehicle crashed in western Shasta County as he worked with crews in dangerous terrain to battle the nearly two-week-old Carr Fire.

Ayeta was the seventh person to die in that blaze, which has scorched more than 62,322 hectares in the Shasta-Trinity region north of Sacramento, including two young children and their great-grandmother whose home was overrun by flames, a resident who authorities said refused to leave his home despite evacuation orders, and two firefighters.

Firefighters had managed to contain 41 per cent of the Carr blaze by Sunday, and authorities were letting some evacuees return.

A team of 188 specialist Australian and New Zealand firefighters are also on the ground in California to help battle the blazes.

This year, California wildfires have burned more land earlier in the fire season than usual, Californian Fire director Ken Pimlott said during a news conference yesterday.

“Fire season is really just beginning. What seems like we should be in the peak of fire season, historically, is really now the kind of conditions we’re seeing really at the beginning,” he said.

Through last week, California fires had torched about 117,359 hectares, more than double the five-year average over that same period, according to Cal Fire.