Winds and temperatures were expected to increase slightly amid a drop in humidity, California officials said in an update early Monday, as the Park Fire had scorched an area greater than the city of Los Angeles.

Nearly 4,000 firefighters are battling the fire, aided by numerous helicopters and air tankers. Reinforcements are expected to give much-needed rest to local firefighters, some of whom have been working non-stop since Wednesday, said Jay Tracy, a Park Fire headquarters spokesperson.

"This fire is surprising a lot of people with its explosive growth," he said. "It is kind of unparalleled."

Firefighters made progress and were aided by improving weather over the weekend in the battle against wildfires covering massive areas and leaving a trail of damage in the western United States, but further evacuations and resources have been necessary as thousands of personnel tackle the flames.

The Park Fire was one of more than 100 blazes burning in the U.S. on Sunday, according to the National Interagency Fire Center. Some were sparked by the weather, with climate change increasing the frequency of lightning strikes as the western U.S. endures blistering heat and bone-dry conditions

The Park Fire spanned more than 1,455 square kilometres of inland northern California. It was started Wednesday, when authorities say a man pushed a burning car into a gully in Chico and then fled. Ronnie Stout, 42, was arrested Thursday and is due in court Monday, accused of setting the fire.

The Park Fire has destroyed at least 66 structures and damaged five others, said Tracy.

WATCH l Odd, eerie scenes of juxtaposition after Jasper fire:

Media allowed to view wildfire aftermath in Jasper, Alta.

Alberta officials say that an estimated one-third of the structures in Jasper are destroyed, including the St. Mary and St. George Anglican Church, built in 1928.

Nearest major city not in threat at present

Although cooler-than-average temperatures are expected through the middle of this week, that doesn't mean existing fires will disappear, said Marc Chenard, a meteorologist at the National Weather Service's Weather Prediction Center in College Park, Md.

The National Weather Service issued "red flag" warnings Monday for wide swaths of Idaho, Montana, Utah and Wyoming, in addition to parts of California, meaning dry fuels and stronger winds were increasing the fire danger, the weather service said.

In California, Paradise and several other Butte County communities were under an evacuation warning Sunday. The Park Fire's southernmost front, which is closest to Paradise, was "looking really good," Cal Fire operations section chief Jeremy Pierce said around midday Sunday. 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 Park Fire has drawn comparisons to the 2018 Camp Fire that tore through Paradise, killing 85 people and torching 11,000 homes.

Fires also in Idaho, Oregon

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 155 square kilometres in four days, Andrew Freeborn of the Kern County Fire Department said.

"When people are trying to ignore the orders and later call for rescue, that takes firefighters away from the task of fighting the fires," said Freeborn. "This fire is moving at a pace and with such intensity that individuals should not be thinking they can wait until the last minute. They need to get out of the way."

The historic mining town of Havilah and several other communities were "heavily impacted" by the fires, but it was too soon to count the burned homes, Freeborn said.

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 111 square kilometres as of Sunday.