Drought-stricken western United States prepared for more wildfire damage on Friday, as efforts to stop a massive inferno raging across southern Oregon faltered, and deadly dry lightning storms are expected in California.

The Bootleg Fire near Oregon's border with California grew overnight to 240,000 acres -- larger than New York City, and by far the biggest active blaze in the US -- while remaining just 7% contained.

"The Bootleg Fire perimeter is more than 200 miles long -- that's an enormous amount of line to build and hold," said firefighter commander Rob Allen, according to AFP inputs.

"We are continuing to use every resource, from dozers to air tankers to engage where it's safe to do so especially with the hot, dry, windy conditions predicted to worsen into the weekend."

Late Thursday, further evacuation orders were issued as firefighters were forced to retreat from fast-moving flames and "severe fire conditions" to the east of the wildfire, which started 10 days ago and has expanded at a rate of 1,000 acres per hour since then.

"I was out there," said Frank Lee Smith, an evacuated resident of Klamath County in Oregon.

"I saw the flames creeping up the side of the bluff about a mile away towards our place and got the call to pack up and go so I threw what I could in the truck and the two dogs and we took off."

The increasing fire also poses a risk to the power supply of neighbouring California's, threatening a blackout, as has happened in past years when heatwaves have strained the state's grid.

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Even while California battles its own flames, Governor of the state Gavin Newsom said that more troops would be deployed to help combat the Oregon wildfire, according to a statement from the governor's office's Emergency Services.

"Climate change impacts are contributing to wildfires that are increasingly dangerous and destructive across the Western US," it added.

Climate scientist Daniel Swain warned that the risk of wildfires ignited by dry lightning strikes forecast in California for this weekend is "quite high."

Last year's August Complex fire -- the largest in modern California history, which destroyed an area the size of Delaware -- was triggered by a massive series of thousands of lightning strikes.

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Due to a "long period of unrelenting and frequently record-breaking heat," California brush is drier than it would usually be at its August or September peak, warned Swain, of University of California, Los Angeles (UCLA).

But it is "very unlikely there will be nearly as many dry lightning strikes as occurred in Aug 2020," he tweeted.

Meanwhile, scientists say heatwaves arriving in the western US and Canada in late June would have been "virtually impossible" without human-caused climate change.

Climate change amplifies droughts that dry out regions, creating ideal conditions for wildfires to spread out-of-control and inflict unprecedented material and environmental damage.