With wildfires in the drought-hit western US and Canada continuing to scorch vast areas, firefighting resources are being severely stretched, authorities have said.

More than 2,100 firefighters are struggling to contain the vast Bootleg Fire raging in southern Oregon, near the border with California.

Some were forced to retreat as the fire spread amid the fourth intense heatwave of the summer.

California, hit by its own wildfires, vowed to send firefighters to help out in Oregon.

And Canada was bringing in some 100 firefighters from Mexico to bolster their exhausted counterparts in northwestern Ontario, provincial authorities announced.

The Bootleg Fire, by far the biggest active blaze in the US, is covering an area larger than New York City and remains just 7% controlled.

It has destroyed at least 20 homes.

Bob Dillon poses for a portrait on the ashes of an extinguished fire that surrounded his home caused by the Bootleg Fire, in Beatty, Oregon

Authorities said the huge blaze had generated ominous "fire clouds" reaching miles in the air, able to spawn dangerous outbreaks of dry lightning and "fire tornadoes."

Firefighter commander Rob Allen noted that the fire's 320km perimeter is "an enormous amount of line to build and hold."

"We are continuing to use every resource, from dozers to air tankers," he said.

"We have had record heat, and just all the worst possible conditions at one time," Suzanne Flory, a US Forest Service spokesperson, told the Oregonian newspaper.

A pyrocumulus cloud from the Bootleg Fire drifts into the air near Bly, Oregon

Meantime, dangerous dry lightning storms were forecast in California.

Last year's August Complex fire, the largest in modern California history, was triggered by a massive series of lightning strikes.

With several areas being evacuated, the US Department of Homeland Security reassured any undocumented immigrants in the area they need not fear seeking assistance.

"Absent exigent circumstances," the department said in a statement, "immigration enforcement will not be conducted at locations where disaster and emergency response and relief is being provided."

In Canada, officials anticipate high temperatures in coming days from Alberta to Ontario, but nothing like the record 49.6C recorded near Vancouver three weeks ago.

But while heat warnings were lifted in some areas, air quality alerts were issued across four western provinces.

More than 300 fires were still raging in western British Columbia, including 15 which started in the past two days, according to the BC fire department.

It said around 30% of those fires were considered out of control.

Scientists say the current heat waves would have been "virtually impossible" without human-caused climate change.