A three-week-old wildfire has engulfed a tiny northern California mountain town, levelling most of its historic downtown and leaving blocks of homes in ashes, while a new wind-whipped blaze also destroyed homes as crews braced for another explosive run of flames.

The Dixie Fire, swollen by dry vegetation and 64km/h gusts, raged through the northern Sierra Nevada community of Greenville on Wednesday night (local time).

A petrol station, church, hotel, museum and bar were among many fixtures gutted in the town, which dates to California’s Gold Rush era and had some structures more than a century old.

The fire “burnt down our entire downtown. Our historical buildings, families homes, small businesses, and our children’s schools are completely lost”, Plumas County Supervisor Kevin Goss wrote on Facebook.

As the fire’s north and eastern sides exploded on Wednesday, the Plumas County Sheriff’s Office issued a warning online to the town’s 800 residents: “You are in imminent danger and you MUST leave now!”

The growing blaze that broke out July 21 is the state’s largest current wildfire and had blackened more than 1300 square kilometres.

The cause is under investigation but Pacific Gas & Electric has said it might have been sparked when a tree fell on one of its power lines.

The fire was burning near the town of Paradise, which largely was destroyed in a 2018 wildfire that became the US’s deadliest in at least a century and was blamed on PG&E equipment.

By Thursday, the Dixie Fire had become the sixth-largest fire in state history, the California Department of Forestry and Fire Protection said. Four of the state’s other five largest wildfires were all in 2020.

No injuries or deaths were reported. Dozens of homes had already burned before the flames made a new run on Wednesday.

After firefighters made progress earlier in the week, red flag weather conditions of high heat, low humidity and gusty afternoon and night-time winds erupted on Wednesday and were expected to be a continued threat.

Heatwaves and historic drought tied to climate change have made wildfires harder to fight in America’s west.

More than 20,000 firefighters and support personnel were battling 97 large, active wildfires covering more than 7500 square kilometres in 13 US states, the National Interagency Fire Centre said.

-AAP