Authorities have warned of mass fatalities as the largest firestorms on record ravaged the states of California and Oregon amid a deadly summer of wildfires.

Deadly infernoes raging across Oregon have kept half a million people under evacuation alert as weary firefighters took advantage of improved weather to go on the offensive against the blazes.

The fires have destroyed thousands of homes in days, making Oregon the latest epicentre in a larger outbreak torching the western United States.

Los Angeles County firefighters, using only hand tools, keep fire from jumping a fire break at the Bobcat Fire in the Angeles National Forest. Photo: Getty

Dozens of people are missing and Oregon’s governor Kate Brown has warned the the death toll could grow far higher, with 25 confirmed dead across the states so far.

Oregon Office of Emergency Management chief Andrew Phelps said disaster teams searching the scorched ruins of a half-dozen small towns laid to waste were bracing to encounter possible “mass fatality incidents.”

Search and rescue personnel from the Jackson County Sheriff’s Office look for the possible remains of a missing elderly resident. Photo: Getty

In one heartbreaking story, CNN reports the body of a 13-year-old boy was found huddled with his dog in a car where they had sought refuge from the flames.

Wyatt Tofte died in the wildfires in Marion County, Oregon, along with his 71-year-old grandmother, Peggy Mosso, a family spokesperson said.

“After a long search for Wyatt, he was found in a car with his dog on his lap, but unfortunately, was not able to escape the fire,” said a family statement to CNN.

“Angie (his mother) is in critical condition with full-body burns.

“We want to thank all emergency personnel and people who helped in the search. Our family appreciates the love and support we have received from everyone during this terrible tragedy.”

Former NSW fire chief Greg Mullins says the wildfires ravaging the US are a 'direct reflection' of what happened in Australia last summer, and serve as another 'wake up call' to pay attention to climate change.#ClimateEmergency https://t.co/miFyG9wICU via @SBSNews

— Lillibet Marie (@lillibet_marie) September 12, 2020

The Pacific Northwest as a whole has borne the brunt of an incendiary onslaught that began around Labor Day, darkening the sky with smoke and ash that has beset northern California, Oregon and Washington with some of the world’s worst air-quality levels.

The firestorms, some of the largest on record in California and Oregon, were driven by high winds that howled across the region for days in the midst of record-breaking heat.

Scientists say global warming has also contributed to extremes in wet and dry seasons, causing vegetation to flourish then dry out, leaving more abundant fuel for wildfires.

-with AAP