Dense smog from US wildfires that have burnt nearly 2.02 million hectares and killed 27 people smothered the West Coast on Saturday, as presidential challenger Joe Biden warned climate change is becoming an existential issue.

A man watches the wildfire in Angeles National Forest, Monrovia, Los Angeles, the United States, Sept. 10, 2020. Wildfires have scorched a record 3.1 million acres (12,525 square km) of land in the U.S. state of California since August, authorities said Thursday.(Photo: Xinhua)

US officials braced for the possibility of further fatalities as wide stretches of land in California, Oregon and Washington remained cut off by flames fueled by tinder-dry conditions of the kind caused by climate change.

Unprecedented infernos have now destroyed an area roughly the size of New Jersey.

"The science is clear, and deadly signs like these are unmistakable - climate change poses an imminent, existential threat to our way of life," said Biden, the Democratic nominee who will face President Donald Trump at the polls on November 3.

"President Trump can try to deny that reality, but the facts are undeniable," he said.

The political row comes as Trump prepares to visit California on Monday for a briefing on the wildfires, which he has previously sought to blame on mismanagement by state officials.

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 from the blazes 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 only as Jessie.

More than 20,000 firefighters are battling the blazes, with officials warning that a respite provided by the arrival of cooler weather could end on Monday with the return of warmer, drier weather.

Oregon suffered another four deaths  on Friday and Saturday, bringing the West Coast region's death toll for last week to 19.

Among them was a 13-year-old boy found in a car with his dog in his lap in Oregon. The road was so hot it had melted the tires as he tried to flee.

Emergency official Andrew Phelps warned Oregon is "preparing for a mass fatality incident based on what we know and the number of structures that have been lost."

Preparations have been hampered by false online claims that "extremists" are intentionally setting fires in Oregon, which have been debunked by the FBI.

In Estacada, some inhabitants had returned from evacuation on Saturday  and patrolled the streets with guns for fear of looting.

Huge wildfires are becoming more common, with the World Meteorological Organization saying the five years to 2019 were unprecedented for fires.