A wildfire that erupted outside California's state capital just a few days ago had exploded to cover 54,000 acres by Wednesday, an eight-fold increase in 24 hours.

At least two people had to be airlifted to hospital as the Caldor Fire tore through a small town around 80 kilometers (50 miles) from Sacramento.

Thousands of people have been told to seek safety, with the blaze raging uncontrolled through the Eldorado National Forest.

"Please, please heed the warnings, and then when you're asked to get out, get out," Fire Chief Thom Porter said, according to the Sacramento Bee newspaper.

"We need you out of the way so we can protect your homes from these fires."

In a stunning demonstration of the way these blazes transform, the Caldor Fire, which began on Saturday, grew from around 6,500 acres on Tuesday morning, fanned by strong winds and huge reserves of tinder-dry fuel.

The Caldor Fire is one of scores raging across the desiccated western United States, as man-made climate change alters weather patterns and brings chronic drought to the region.

On Wednesday, the Lake County Sheriff's Office issued evacuation orders for parts of the town of Lower Lake, including two schools and a mobile home park, after a new blaze erupted there.

"If you're in Lower Lake, you should probably get out of here," Sheriff Brian Martin said in a Facebook video.

"We've got some pretty sticky situations going on. This is a very, very serious event."

The Lake County News said firefighters had seen RVs ablaze, with reports that authorities were going door-to-door to urge people to flee the quickly developing Cache Fire.

- Dixie Fire still burning -

Further north, the huge Dixie Fire continued to burn.

It has now scorched more than 600,000 acres in the month since it started, and is the second biggest blaze in California's history.

Photos taken by an AFP journalist show towering flames consuming the trees along the side of a highway, as firefighters try to set containment lines.

In the town of Janesville, the burned out hulks of cars sit among the still-smoking undergrowth; elsewhere, mailboxes exposed to the ferocious heat of the flames have melted out of shape.

The acrid smoke created by wildfires sparked an air quality advisory Wednesday for residents in and around San Francisco.

Utility PG&E on Tuesday began shutting off power supply to more than 50,000 customers.

The company -- which has acknowledged that its equipment may have started the Dixie Fire -- said the shut-offs were to avoid the danger of live lines falling on dry vegetation.

The last decade has seen a huge rise in the number of wildfires in the west of the United States.

Climate change linked to the burning of fossil fuels has made the region dryer and hotter for longer, creating ideal conditions for the blazes to rage.