The fires now cover an area of 802 square kilometres; to give that some physical context, collected, it is an area larger than all five boroughs of New York City, or almost half of Sydney's urban area.

The main fire in northern California, the "Camp" fire, has burned through 45,729 hectares and is now reporting 25 per cent containment.

The largest fire in southern California, the "Woolsey" fire, has burned 37,057 hectares and authorities say it is just 20 per cent contained.

"Today was a better day," LA county fire chief Daryl Osby said. "As I stand here at this moment it's encouraging that none of the flare-ups exceeded the containment lines."

Nonetheless authorities urged those sheltering in place to evacuate, and evacuees not to return to their homes, as dry conditions and worsening winds could easily reverse the gains.

The Camp fire, which accounts for 29 of the 31 dead, is the worst fire for fatalities in the state's history.

More than 8000 firefighters are deployed battling the fires, supported by a fleet of more than 70 water tenders and 42 helicopters.

Much of the gained ground overnight was owed to the air fleet, which focused on the creation of new containment lines, making it as difficult as possible for the fire to spread.

Firefighting support has been drawn in from nearby states including Oregon, Washington, Utah, New Mexico and Texas.

Authorities say around 72,000 structures remain under threat.

Some areas in southern California have been reopened to residents, including parts of Thousand Oaks and Agoura, but have warned residents to remain vigilant in case conditions worsen again.

The Los Angeles Country Sheriff's Department said they would maintain a "strong presence in repopulated and evacuated areas" to protect residents and property.

California governor Jerry Brown said the state's bushfire threat had fundamentally changed in recent years.

"This is not the new normal, this is the new abnormal," Governor Brown said.

"This new abnormal will continue certainly in the next 10 to 15 to 20 years," Brown added.  "The best science is telling us that dryness, warmth, drought, all those things, they’re going to intensify.

"We have a real challenge here threatening our whole way of life," Brown said.

The National Weather Service has forecast worsening conditions until later this week: low humidity and strong, gusty Santa Ana winds.

"The combination of strong winds and extremely low humidity will create a dangerous fire weather environment during this period," a spokesman for the weather service said. "Any fires that develop will likely spread rapidly with extreme fire behaviour."

Meanwhile, US president Donald Trump shifted the tone of his commentary on the fires, reacting to fierce criticism across the political spectrum for the insensitivity of his first remarks.

Those comments - Trump blamed California for the fires, despite the fact that 60 per cent of the state's forests are under federal control - were challenged by firefighter union president Brian Rice.

Trump's comments were "ill-informed, ill-timed and demeaning to those who are suffering as well as the men and women on the front lines," Rice said.

"The California firefighters, FEMA and first responders are amazing and very brave," Trump said on social media. "Thank you and God Bless you all."

FEMA - the Federal Emergency Management Agency - is the part of the Department of Homeland Security which responds to national disasters.