The Camp fire, in northern California, has burned more than 42,000 hectares, destroyed almost 7000 structures and accounts for 23 of the 25 reported fatalities.

Two more fatalities were reported on the weekend in the Woolsey fire; in addition, three firefighters have been injured.

Though there are conflicting reports, at least 100 people are still listed as missing.

Authorities said the bodies of the dead are being recovered "with as much dignity as we can afford them."

The Camp fire is now 20 per cent contained, a significant gain given the fire was burning out of control, reducing the entire town of Paradise, California, to cinders just 24 hours earlier.

That small gain over the Camp fire may prove critical in the days ahead as the National Weather Service has forecast brutal conditions: low humidity and strong, gusty Santa Ana winds until at least Wednesday, Australian time.

"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."

More than 7200 firefighters are now deployed battling the three major fire fronts, supported by 73 water tenders, 42 helicopters and almost 200 support crew and 135 bulldozers.

Authorities say around 72,000 structures remain under threat.

The fire has already set a record as the worst in California's history, but unprecedented also is the extent of the evacuations: a quarter of a million people have been displaced by the disaster, seeking shelter either with relatives, friends or in a number of shelters managed by disaster relief services.

Some, including the actor Martin Sheen and his wife, slept in their car at Zuma Beach, a sliver of the Malibu coastline where many evacuees have gathered. Others abandoned their cars on the roads when fierce fire conditions melted their tyres.

In addition to fighting the fires, emergency services attempted to solidify defensive lines against the fires ahead of the worsening conditions.

"We have fuels that are in critical drought state right now," a spokesman for Cal Fire told media. "This is the sixth year of drought in this region. Our firefighters have been facing some extreme, tough fire conditions that they said that they've never seen in their lives."

As social media was flooded with video footage of residents fleeing their homes through firestorms, and on highways lined by walls of fire and raining embers, a war of words erupted between US president Donald Trump and emergency responders who criticised the president for threatening to withdraw federal aid.

Trump, who is travelling in Europe, had tweeted a threat to withdraw federal payments to California, alleging that the state's forest management is "poor."

"The president's message attacking California and threatening to withhold aid to the victims of the cataclysmic fires is ill-informed, ill-timed and demeaning to those who are suffering as well as the men and women on the front lines," president of the California Professional Firefighters union Brian Rice said.

"At this moment thousands of our brother and sister firefighters are putting their lives on the line to protect the lives and property of thousands, some of them doing so even as their own homes lay in ruins," Rice added.

He described the president's words as "an attack on all our courageous men and women on the front lines."

Rice challenged the president's assertion that California had poorly managed its forests, pointing out that nearly 60 percent of California forests are under federal management.

"It is the federal government that has chosen to divert resources away from forest management, not California," Rice said. "Wildfires are sparked and spread not only in forested areas but in populated areas and open fields fuelled by parched vegetation, high winds, low humidity and geography."

The cause of the fire is still officially "under investigation", though the Pacific Gas & Electric Company has notified state regulators that one of its high-voltage power lines located near the northern California fire front malfunctioned shortly before the first fires broke out.