Bootleg is the largest of 80 major fires now active in the US. According to the officials, the wildfire spread overnight from 274,000 acres to 290,000 acres -- three times the size of the metropolis of Detroit. Nearly 2,000 people have had to evacuate the area and more are expected to follow on Sunday.
Satellite imagery from the National Weather Service showed a huge plume of smoke soaring from Bootleg, in southern Oregon, to the Canadian border, hundreds of miles to the northeast.
But, with firefighters making progress on Bootleg's western flank, overall containment of the blaze has more than tripled, to 22%, though heavy winds and widespread lightning storms remained a serious threat.
In California's Lake Tahoe, firefighters are blaming lightning strikes for a fast-growing fire in the tourist area. Dubbed as Tamarack Fire, the blaze has grown explosively to more than 20,000 acres, with zero containment so far.
The small nearby community of Markleeville, on the Nevada border, has been evacuated.
Scientists say climate change amplifies droughts which create ideal conditions for wildfires to spread.
The National Interagency Fire Center said the outlook was for "very hot, dry and unstable conditions across the inland Pacific Northwest, Northern Rockies and Plains into northern Minnesota."
It said nearly 20,000 firefighters and support personnel are struggling to contain fires raging across the Western states, with more than 2.5 million acres already having burned this year.
Firefighters in Canada, meanwhile, continued to battle dozens of blazes, including some 20 new ones in British Columbia province and about 15 new ones in northwest Ontario province.
Authorities in that province said a firefighter had died in hospital of an unspecified "medical emergency."