US President Donald Trump on Monday further proved that he has no idea how climate change or forest fires work.

In a new, false and unscientific claim, he said that 'trees just explode,' on why the US wildfires which are raging on unabated, actually started.

Trump repeated his argument that wildfires are caused by poor maintenance of forest areas, making them more combustible.

"There has to be strong forest management," he said.

"With regard to the forests, when trees fall down after a short period of time, about 18 months, they become very dry. They become really like a match stick," he added. "They just explode."

Infernos across California, Oregon and Washington state have burned more than five million acres (two million hectares) this year, killed dozens of people and forced hundreds of thousands from their homes.

Whole towns have been ruined by the blazes, which have pumped out tonnes of noxious fumes that have hung in the air for days, blocking out the sun and making breathing difficult.

But Trump, in Sacramento on the third day of a re-election campaign swing, pushed back against state officials arguing that a heating climate underlies the ever-stronger fires.

"It will start getting cooler. You just watch," Trump said, offering no evidence for the theory.

"I wish science agreed with you," responded Wade Crowfoot, the head of the California Natural Resources Agency.

"I don't think science knows, actually," Trump said.

Reputable scientists around the world are almost unanimous in their belief that the world is getting warmer because of human activity.

This man-made climate change amplifies droughts, which dry out regions, creating ideal conditions for wildfires to spread out-of-control and inflict unprecedented damage.

Much of the West Coast has been blanketed in choking smog for days, with Portland on Monday the world's most air-polluted city according to IQAir.

Washington state Governor Jay Inslee said it was crucial voters elect a "climate president" come November 3.

"This moment requires action, not denial," he said in a tweet. "We must come together around science to build a better future for all Americans."

Most of the 35 deaths so far confirmed have occurred in California and Oregon, where the wildfires are the worst in decades and have been fueled partly by record heat.

More than 30,000 firefighters are battling the blazes, with wind gusts and drier weather on Monday threatening more destruction.

Residents of Arcadia, on the outskirts of Los Angeles, were ordered to evacuate Sunday as the nearby Bobcat Fire spread south through forested terrain toward the metropolis.

Two new deaths were confirmed from the North Complex Fire, which swept at unprecedented speeds last week into areas already ravaged less than two years ago by the Camp Fire -- California's deadliest ever blaze.

"There are still active fires, power lines are down, trees are down, there are roads that are impassable," said Butte County Sheriff Kory Honea, warning evacuees it could take "weeks and weeks" to return to their homes.

(With inputs from AFP)