Emergency services in the US are fighting misinformation along with raging wildfires as people spread unsubstantiated social media posts blaming coordinated groups of arsonists from both the far left and far right for setting the blazes.

The FBI said it had investigated several claims and found them to be untrue, while officials in Oregon and Washington state turned to Facebook to knock down the competing narratives.

Some posts blamed far-left antifa activists and others claimed the far-right group the Proud Boys was responsible for fires which have killed at least 24 people across Oregon, Washington and California.

"I am physically and emotionally exhausted. We've been working really hard to protect people's lives and homes," firefighter Matt Lowery wrote on the Facebook page for the East Pierce Fire & Rescue union south of Seattle.

"I also want to address an issue that keeps coming up, even from some of the public that we are talking to while working. It is hot, dry, and fire spreads quickly in those conditions. There is nothing to show it's antifa or Proud Boys setting fires. Wait for information."

The Mason County Sheriff's Office urged Washington residents to stop spreading rumours as isolated incidents of apparent arson led to widespread, unfounded claims that antifa agitators were conspiring to start fires along the West Coast.

Antifa is short for anti-fascists, a range of far-left militant groups that oppose white supremacists.

"Though some agencies have made arrests related to arson recently, they appear to all be separate individuals, however as with many incidents, it will be an ongoing investigation in each jurisdiction," the agency wrote on Facebook.

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While some arson arrests have been made, it is not yet clear how all the scores of fires in Washington state and Oregon started.

Officials say high winds and dry conditions have made them worse in a region with a cool, wet climate that has historically protected it from intense fire activity.

Both Oregon Governor Kate Brown and Washington Governor Jay Inslee have called the wildfires "unprecedented" while California Governor Gavin Newsom has said the deadly, record-shattering fire season across the US West should end all debate over climate change.

Left and right-wing groups have clashed during protests in the region, particularly in Portland, Oregon, where a caravan of US President Donald Trump's supporters drove cars through the liberal city last month.

An antifa supporter shot and killed a member of a right-wing group and was himself fatally shot by Washington state authorities a week later.

The FBI said it worked with local authorities to investigate claims that extremists set wildfires and found them to be false.

"Conspiracy theories and misinformation take valuable resources away from local fire and police agencies working around the clock to bring these fires under control," an FBI statement said.

"Please help our entire community by only sharing validated information from official sources."

Officials in Oregon also debunked claims of widespread arrests of people affiliated with the Proud Boys or antifa.

The surge of misinformation in the United States comes after a similar experience in Australia during the bushfire season in late 2019 and early 2020.

The ABC found suspicious accounts amplified incorrect claims that arson was the overwhelming cause of the fires.

One area of misinformation on Twitter was the hashtag #ArsonEmergency, which a Queensland University of Technology researcher found was attracting a "suspiciously high number of bot-like and troll-like accounts".

Among the unsubstantiated claims, some posts wrongly asserted that hundreds of arsonists had been arrested, and that some of the fires had been started deliberately by left-wing and environmental activists.

Misleading photos and inaccurate fire maps were widely shared, including by international celebrities.

In June, the royal commission into the bushfires heard that while social media and community noticeboards were the most effective communication methods during an emergency, they could also be used to quickly spread misinformation.

In general, experts recommend using trusted media outlets, checking multiple sources and avoiding sharing content without a clear attribution.

AP/ABC

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Posts on social media are blaming antifa and the Proud Boys for the US wildfires. Firefighters, police and the FBI say it isn't true - ABC News