PORTLAND (Oregon) • Deadly wildfires raging across Oregon have kept half a million people under evacuation alert even as weary firefighters took advantage of improved weather to go on the offensive against the blazes on Friday.

The fires have destroyed thousands of homes in days, making Oregon the latest epicentre in a larger summer outbreak of fires sweeping the West Coast of the United States, killing at least 25 people and collectively scorching a landscape the size of New Jersey.

At least five people died in Oregon this past week, and Governor Kate Brown has warned that the death toll could be far higher. She said on Friday that dozens of people had been reported missing in three counties in the state.

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 themselves to encounter possible "mass fatality incidents".

The Pacific North-Western region as a whole has borne the brunt of an incendiary onslaught that began around Labour Day, last Monday, darkening the sky with smoke and ash that has beset northern California, Oregon and Washington state 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.

"This is a climate... emergency. This is real and it's happening. This is the perfect storm," California Governor Gavin Newsom told reporters from a charred mountainside near Oroville in the state.

More than 3,900 homes and other structures have been incinerated in California alone over the past three weeks.

In southern Oregon, an apocalyptic scene of charred residential subdivisions and trailer parks stretched for kilometres along Highway 99 south of Medford through the neighbouring towns of Phoenix and Talent, one of the most devastated areas.

Molalla, a community about 40km south of downtown Portland, was an ash-covered ghost town after its more than 9,000 residents were told to evacuate, with only 30 refusing to leave, the city's fire department said.

The logging town was on the front line of a vast evacuation zone stretching north to within 5km of downtown Portland. The sheriff in suburban Clackamas County has set a 10pm curfew to deter "possible increased criminal activity".

Oregon's Governor Brown told a news conference that more than 500,000 people were under one of three evacuation alert levels, advising them to pack and be vigilant, and to be ready to flee at a moment's notice, or to leave immediately. About 40,000 of those had already been ordered to leave.

In neighbouring Washington state, online video from the Tacoma area showed fires in a residential area setting homes ablaze and locals scurrying to warn their neighbours.

"Everybody out, everybody out!" a man screamed as firefighters tried to douse the flames.

After four days of treacherously hot, windy weather, a glimmer of hope arrived on Friday in the form of calmer winds blowing in from the ocean, bringing cooler, more moist conditions that helped firefighters make headway against blazes that had burned largely unchecked earlier in the week.

"The weather is going to be favourable for us," said Mr Doug Grafe, fire protection chief for the Oregon Department of Forestry, adding that the break in the weather was forecast to continue into this week.

The overall death toll from the West Coast fires that began last month jumped to 25 after seven people were reported killed in mountains north of Sacramento, California, and Oregon's fifth fatality was reported in Marion County, outside of Salem, the state capital.

Meanwhile, Paradise, a town devastated by California's deadliest wildfire in 2018, has posted the world's worst air quality index reading at 592, according to the PurpleAir monitoring site, as two of the state's largest blazes burned on either side of it.

In southern Oregon, police arrested a 42-year-old man on Friday for starting a fire in the town of Phoenix, the Jackson County Sheriff's office said.

The suspect, identified as Michael Bakkela and described as a "local transient", has been charged with arson, criminal mischief and reckless endangering, the office said in a press release.

REUTERS