Democratic presidential nominee Joe Biden has called President Donald Trump a "climate arsonist" for failing to acknowledge the role of global warming in the Western wildfires, while Trump said forest management was the key to controlling the blazes. 

Wildfires across Oregon, California and Washington state have destroyed thousands of homes and a half-dozen small towns since August, scorching more than 4 million acres (1.6 million hectares) and killing more than two dozen people.

Biden, slammed by Republicans for not visiting disaster areas, spoke from his home state of Delaware on the threat of extreme weather that climate scientists have said is supercharging fires.

The Republican president, who trails Biden in national polls ahead of the Nov. 3 election, met with firefighters and officials in California after Democrats blasted the president for remaining mostly silent about the largest wildfires in state history.

"I think this is more of a management situation," Trump said, when asked by a reporter if climate change was a factor behind the fires, saying many other countries were not facing a similar problem. He said forest management changes were something that could be tackled quickly, whereas climate change would take more time.

"When you get into climate change, well is India going to change its ways? And is China going to change its ways? And Russia? Is Russia going to change its ways?" he said after landing in McLellan Park, California.

Trump has referred to climate change as a "hoax' and in 2017 pulled the United States out of the Paris accord that laid out an international approach to global warming, while Biden says climate change is on his list of major crises facing the United States.

Calling Trump a "climate arsonist," Biden said: "If we have four more years of Trump's climate denial, how many suburbs will be burned by wildfires? How many suburban neighborhoods will have been flooded out?"

California Governor Gavin Newsom acknowledged that his state had not done enough to manage forests and has acknowledged that over 100 years of fire suppression has allowed fuel to build up.

But he said global warming was driving fires, reminding Trump that 57 percent of forests in the state were under federal management.

"We come from a perspective humbly where we submit the science is in and observed evidence is self-evident: that climate change is real, and that is exacerbating this," the Democratic governor said during a meeting with the president.

Trump, who has authorized federal disaster aid for both California and Oregon, questioned that science.

"It'll start getting cooler, you just watch," he said. "I don't think science knows."

The heated words and denial come as US government scientists  says this summer was the hottest ever recorded in the northern hemisphere.

June, July and August were 1.17C (2.11F) above the 20th-century average, according to the National Oceanic and Atmospheric Administration (Noaa).

CNN meteorologist Brandon Miller said that it is no coincidence that California, Oregon and Washington are having record-shattering fire years in the year that the Northern Hemisphere temperatures are at their hottest.

"That is because climate change is making heat waves hotter and more frequent. Rising temperatures are making droughts and fires more widespread. And the Northern Hemisphere has a large majority of the Earth's land and population. So the worst impacts tend to occur during the time when the Northern Hemisphere temperatures are the highest," he said.