The massive Dixie Fire, which started July 14, had already levelled more than a dozen houses and other structures when it combined with the Fly Fire and tore through the tiny northern California community of Indian Falls after dark on Saturday.

An updated damage estimate was not available on Sunday, though fire officials said the blaze had charred more than 770 square kilometres of timber and brush. It was 21 per cent contained.

Firefighters carrying hand tools were forced to hike through rugged terrain where engines could not go, said Rick Carhart, spokesman for the California Department of Forestry and Fire Protection.

Still, crews made progress on Saturday by proactively setting fires to rob the main blaze of fuels, he said.

Firefighters also reported progress against the nation's largest wildfire, the Bootleg Fire in southern Oregon, containing 46 per cent of the blaze that had consumed nearly 1660 square kilometres.

More than 2200 firefighters battled the blaze, focusing on Sunday on building containment lines at the north and eastern edges.

Crews could get a break from rain and higher humidity predicted for this week, said Marcus Kauffman, spokesman for the Oregon Department of Forestry.

Oregon Governor Kate Brown told CNN it was imperative federal and state authorities invested in mitigation such as tree thinning and preventive burns to limit the number of similar massive blazes.

But she conceded "the harsh reality is that we're going to see more of these wildfires".

"They're hotter, they're more fierce, and obviously much more challenging to tackle. And they are a sign of the changing climate impacts," Brown said on Sunday.

More than 85 large wildfires were burning around the country, most of them in western states including Washington and Utah, and had burned more than 553,000 hectares.

Australian Associated Press