The Dixie Fire tore through the Greenville on Wednesday evening, destroying businesses and homes as the sky was cast in an orange glow.

"If you are still in the Greenville area, you are in imminent danger and you MUST leave now!!" the Plumas County Sheriff's Office posted on Facebook earlier on Wednesday.

The three-week-old fire has grown to more than 1100 square kilometres across Plumas and Butte counties.

Firefighters had been trying to protect the town northeast of San Francisco by clearing debris from roads and marking hazards.

The destruction came amid a red flag warning issued by forecasters warning of hot, bone-dry conditions with winds up to 64km/h.

"I think we definitely have a few hard days ahead of us," Shannon Prather of the US Forest Service said.

Firefighters were able to save homes and hold large stretches of the blaze. But flames jumped perimeter lines in a few spots Tuesday, prompting additional evacuation orders for about 15,000 people east of Lake Almanor, fire officials said.

Heat from the flames created a pyrocumulus cloud, a massive column of smoke that rose 10,000 metres, said Mike Wink, a state fire operations section chief.

The fire has threatened thousands of homes and destroyed 67 houses and other buildings since breaking out July 14. It was 35 per cent contained.

About 250 kilometres to the west, the lightning-sparked McFarland Fire threatened remote homes along the Trinity River in the Shasta-Trinity National Forest. The fire was only five per cent contained.

Similar risky weather was expected across southern California for much of the week.

Heatwaves and historic drought tied to climate change have made wildfires harder to fight in the American west.

More than 20,000 firefighters and support personnel were battling 97 large, active wildfires covering 7560 square kilometres in 13 US states, the National Interagency Fire Center said.

Montana on Tuesday had 25 active large blazes, followed by Idaho with 21 and Oregon with 13. California had 11.

Australian Associated Press