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

More than 2,100 firefighters were again 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 274,000 acres -- an area larger than New York City -- and remains just seven percent controlled, according to the Inciweb website.

"(The) fire remains very active with significant acreage increases due to hot, dry, and breezy conditions," the official site said.

It has destroyed at least 20 homes.

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

Firefighter commander Rob Allen noted that the fire's 200-mile (320-kilometer) 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 Friday.

"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.

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 -- though nothing like the record 121 degrees Fahrenheit (49.6 degrees Celsius) recorded near Vancouver three weeks ago.

Air quality alerts were issued across four western provinces.

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