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 348,000 acres of land north-east of Chico, and was 0% contained on Saturday, the state's fire agency Cal Fire said.

About 2,500 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 has consumed an area more than 1.5 times the size of New York City’s five boroughs.

Cal Fire incident commander Billy See said the fire has been spreading 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 15F, but still hover in some areas in the low 90s (32C).

Officials said fire whirls are less likely today, after a rare “firenado” – a swirling vortex of flames and ash formed in intense heat and high winds – was filmed twisting through bushland on Friday.