An enormous wildfire has grown by 8 sq miles an hour (20 sq km) as it spreads across parts of northern California.

The Park fire, which started on Wednesday in a suspected arson attack, has burned more than 350,000 acres of land north-east of Chico, and was only 10% contained as of Sunday morning, the state's fire agency Cal Fire said.

About 3,700 firefighters are battling the blaze, which has been fuelled by steep terrain and wind gusts.

A 42-year-old man was arrested on Thursday on suspicion of starting the fire by rolling a burning car into a gully near Alligator Hole in Butte County.

It is now the largest fire in the state this year, and the seventh largest ever recorded in California. It has consumed an area more than 1.5 times the size of New York City’s five boroughs.

Fire captain Robert Foxworthy said steep terrain and very few roads have made it a challenge to battle the blaze. He said the sheer scale of the fire was made worse by embers being flung out a "great distance" ahead of the main fire, sparking a large number of spot fires.

Cal Fire incident commander Billy See said the fire had spread at a rate of 5,000 acres an hour.

Speaking at an operational briefing, Mr See said there were almost three times the personnel fighting the fire on Saturday compared to Friday, and “we still don’t have enough”.

Scott Weese, a fire behaviourist with Cal Fire, said that there was a high fuel load in the area with an abundance of grass.

“The heat signature is huge,” he told the operational briefing, adding that the fire burned through 150,000 acres yesterday.

Authorities were hopeful they could use Saturday's easing of conditions to contain some of the blaze.

Wind speeds decreased and temperatures dropped by about 3-4C, but still hover in some areas in the low 30s (90F).

Cal Fire said lower temperatures and higher humidity have reduced the spread of the blaze but it is still growing in some areas.

On Saturday, officials said the chance of fire whirls had decreased after a rare “firenado” – a swirling vortex of flames and ash formed in intense heat and high winds – was filmed twisting through bushland the day before.

California Governor Gavin Newsom on Friday declared a state of emergency in Butte and Tehama counties because of the Park fire.

On Saturday, the governor announced new federal assistance for Tehama County. Federal assistance for Butte County was announced earlier in the week.

“This is already one of the biggest fires in California history, and we're continuing to see dangerous conditions – our firefighters and emergency responders are working day and night to protect our communities. Californians must heed warning from local authorities and take steps to stay safe."

At least 16 helicopters are fighting the blaze, as well as multiple air tankers dumping water from the sky when conditions allow.

The wildfire has forced mandatory evacuations in Butte, where California's deadliest blaze, the Camp Fire, killed more than 80 people in 2018.

The 400-strong population of Cohasset has already been moved as the fire burns out of control.

Cal Fire said that 134 structures had been destroyed, while 4,200 were under threat.

• First images of historic tourist town Jasper after 100m high wildfire hit

Officials arrested Ronnie Dean Stout, 42, and accused him of "calmly leaving the area by blending in with the other citizens who were in the area and fleeing the rapidly evolving fire" that he had set.

He is being held in jail without bail as authorities determine what charges he will face.

A woman who answered the door of the mobile home listed as his home address in Chico told the San Francisco Chronicle that prosecutors "are trying to make him the scapegoat".

“They’re saying he did it intentionally, but he didn’t. The car caught on fire,” the unidentified woman said, before refusing to answer further questions.

The Park fire is one of many currently burning across the US and Canada.

The National Interagency Fire Center is currently monitoring 102 large fires in the US, mostly in states on the west coast.

In Oregon, a firefighting pilot was killed in a tanker plane crash after the aircraft went missing on Thursday night.

In Canada, a large wildfire in the Jasper National Park destroyed hundreds of buildings in the town of Jasper.

Weekend conditions have improved in Jasper, with rain and cooler temperatures helping efforts to protect other parts of the historic tourism town.