With gusting winds, triple digit temperatures and almost zero chance of rain in Northern California this week, scorching weather poses a persistent threat to firefighters battling out-of-control blazes on parched land, officials said.

"Unfortunately, they're not going to get a break anytime soon," said Brian Hurley, a meteorologist with the National Weather Service's Weather Prediction Center in College Park, Maryland.

"It's pretty doggone hot and dry and it's going to stay that way," Hurley said early Monday.

Some areas in Central and Northern California could see 110 degrees Fahrenheit (43 Celsius) and winds of 15 mph (24 kph) with higher gusts that could fan the flames and spread embers, he said.

A massive, out-of-control Northern California wildfire called the Mendocino Complex Fire that destroyed 68 homes and forced thousands to flee has become the fifth largest in state history, officials said on Sunday, as crews battled high temperatures and strong winds.

It has charred more than 266,900 acres (108010.6 hectares) as of late Sunday, making it the fifth largest blaze in California's history, officials said.

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

Trump said on his official Twitter account Sunday, "California wildfires are being magnified & made so much worse by the bad environmental laws which aren't allowing massive amount of readily available water to be properly utilized."

Another fire, the nearly two-week-old Carr Fire, claimed another life Saturday, a 21-year-old apprentice lineman, Jay Ayeta, officials with the PG&E Corporation said on Sunday.

Ayeta died when his vehicle crashed as he worked with crews in dangerous terrain to battle wildfire in Shasta County.

He was the seventh person to die in that blaze, which has scorched more than 160,000 acres in the scenic Shasta-Trinity region north of Sacramento, including two young children and their great-grandmother whose home was overrun by flames, and two firefighters.

Firefighters had managed to contain 43 per cent of the Carr blaze by late Sunday, and authorities were letting some evacuees return.