Firefighters have made progress partly thanks to improving weather in the battle against wildfires covering massive areas and leaving a trail of damage in the western United States.

Further evacuations and resources have been necessary as thousands of personnel tackle the flames.

The so-called Park Fire, the largest wildfire in California this year, was one of more than 100 blazes burning in the US on Sunday, according to the National Interagency Fire Centre.

Some were sparked by the weather, with climate change increasing the frequency of lightning strikes as the western US endures blistering heat and bone-dry conditions.

A firefighter from the city of Monterey monitors flareups from the Park Fire
Weather conditions have helped in the fight against the fire (AP)

The Park Fire had scorched an area greater than the city of Los Angeles as of Sunday, darkening the sky with smoke and engaging thousands of firefighters.

The blaze spanned more than 562 square miles of inland Northern California.

Millions of people were under air quality alerts on Sunday in the north-western US and western Canada.

The Park Fire started on Wednesday, when authorities say someone pushed a burning car into a gully in Chico and then fled. A man accused of setting the fire was arrested on Thursday and is due in court on Monday.

Firefighters increased containment to 12% on Saturday, aided by cooler temperatures and more humidity, officials said.

Although cooler-than-average temperatures are expected through the middle of this week, that does not mean existing fires will disappear, said Marc Chenard, a meteorologist at the National Weather Service’s Weather Prediction Centre in Maryland.

The Park Fire burns along Highway 32
Firefighters have been tackling the blaze non-stop, authorities said (AP)

A “red flag” warning was in effect for the region on Sunday, meaning dry fuels and stronger winds were increasing the fire danger, the weather service said.

The National Weather Service also issued a “red flag” warnings Sunday for wide swathes of Idaho, Montana, Utah, Colorado and Wyoming, in addition to parts of California.

Fires burned across eastern Oregon and eastern Idaho, where officials were assessing damage from a group of blazes called the Gwen Fire, which was estimated at 43 square miles as of Sunday.

In California, Paradise and several other Butte County communities were under an evacuation warning Sunday. Yet the fire’s southernmost front, which is closest to Paradise, was “looking really good,” Cal Fire operations section chief Jeremy Pierce said around midday.

Officials did not expect it to move farther into Chico, a city of about 100,000 people just west of Paradise, and over the next three days crews plan to extinguish hot spots and remove hazards, Pierce said.

The focus on saving lives and endangered property has shifted to confronting the blaze head-on, according to Jay Tracy, a Park Fire headquarters spokesperson.

Nearly 4,000 firefighters are battling the fire, aided by numerous helicopters and air tankers. Reinforcements are expected to allow rest periods for local firefighters, some of whom have been working non-stop since Wednesday.

The Park Fire has drawn comparisons to the 2018 Camp Fire that tore through Paradise, killing 85 people and torching 11,000 homes.

In Southern California, about 2,000 people were ordered to evacuate because of a fire sweeping through the Sequoia National Forest. The wind-driven blaze was fed by dried, dead plants and moving fast, eating up more than 60 square miles in four days, Andrew Freeborn of the Kern County Fire Department said.

No fatalities have been reported in the Park and Borel fires, but some people are increasing the danger for everyone by disregarding evacuation orders, Mr Freeborn said.