Portland-US President Donald Trump said he will meet emergency services in California on Monday as record fires that have killed 31 people smothered the West Coast in dense smog.

Officials warned of further fatalities with swathes of land in California, Oregon and Washington still cut off by flames fueled by tinder-dry conditions, as presidential challenger Joe Biden linked the fires to climate change.

The unprecedented infernos have now burnt through nearly 5 million acres (2 million hectares), an area roughly the size of New Jersey.

Trump plans to meet with the heads of California’s emergency services, battling fires that have scorched more than 3.2 million acres in the state this year -- an annual record, with nearly four months of fire season still to come.

He has made little comment about the blazes in recent weeks, but at a Nevada campaign event on Saturday he acknowledged the scope of the disaster.

“They never had anything like this,” Trump said. “Please remember the words, very simple, forest management.”

But Biden, speaking earlier, said the causes were much broader: “The science is clear, and deadly signs like these are unmistakable -- climate change poses an imminent, existential threat to our way of life.” “President Trump can try to deny that reality, but the facts are undeniable.” Worsening the sense of environmental catastrophe, all five of the world’s most air-polluted cities Saturday were on the West Coast, according to IQAir, with dense smog and ash coating the atmosphere from Los Angeles up to Vancouver in Canada.

In Portland, thick, choking smoke blanketed the downtown area Saturday morning.

“It is as if I had smoked 100 cigarettes. I’ve never seen this but we try to stay positive as conditions are getting better,” said a 37-year-old man who gave his name as Jessie.