Media captionExperts say this has been the worst start to the fire season in 10 years
Two children and their great-grandmother are among five people to have died in a raging wildfire in northern California, reports say.
Two firefighters died on Thursday, 17 people are missing and tens of thousands have fled their homes.
The fires in Shasta county are being sucked up by strong winds to form "fire tornados" that are uprooting trees and overturning cars, fire officials say.
Firefighters are battling the blaze, which is only 5% contained so far.
The blazes, known as the Carr fire, have destroyed at least 500 structures and are threatening thousands of homes.
The wildfire began on Monday after a car malfunctioned. It has scorched over 48,000 acres (194 sq km) of land - an area larger than the city of San Francisco.
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Sherry Bledsoe has confirmed that her grandmother Melody Bledsoe, 70, and her two children Emily Roberts, five, and James Roberts, four, died in the fire, reports say.
They were caught in the path of the fire as they were about to evacuate their home in the town of Redding, NBC reported.
Melody Bledsoe's husband, Ed, earlier described how she had called him while he was out shopping and told him to return home because the fire was getting close to the house.
When he reached home he found it destroyed and surrounded by police tape, he said.
Another relative told NBC that Melody Bledsoe had called police to say they were trapped inside the house but the line went dead during the call.
Two firefighters - fire inspector Jeremy Stoke, and a bulldozer operator who has not yet been named, died trying contain the blaze.
Media captionWildfire rips through city in northern California
More than 3,400 firefighters have been deployed - but the local fire department has warned that hot, dry weather is forecast for the rest of the week, and could make the blaze worse.
"We are seeing fire whirls - literally what can be described as a tornado," California department of forestry and fire protection (CalFire) chief Ken Pimlott told reporters.
"This fire was whipped up into a whirlwind of activity" by gale-force winds, he said, "uprooting trees, moving vehicles, moving parts of roadways."
"These are extreme conditions... we need to take heed and evacuate, evacuate, evacuate."
What are fire whirls?
Media caption'Fire whirls' spotted in Arizona
Fire whirls, also known as fire "tornadoes", are spinning vortexes of air, ash and fire
They form when rising hot air begins to rotate and forms a vortex that picks up flammable gases and burning debris vegetation
Fire whirls typically only last a few minutes but can be very dangerous because they can move quickly
They can reach dozens of metres in height, with core temperatures as high as 1,090C.
About 37,000 residents have been forced to leave the area.
One local, Liz Williams, found herself and her two children stuck in traffic as people rushed to evacuate. She eventually fled by foot.
"I've never experienced something so terrifying in my life," she told AP news agency. "I didn't know if the fire was just going to jump out behind a bush and grab me and suck me in."
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Image caption
About 500 buildings have been destroyed by the Carr fire
The Carr fire is one of almost 90 active large fires in the US, according to the National Interagency Fire Center.
In California, the Ferguson fire has killed one firefighter and led to the closure of much of Yosemite National Park, while the Cranston fire in Riverside County in the south has burned 11,500 acres (46 sq km) of land.
Wildfires are a common occurrence in California during the state's long, hot, dry summers.
However, experts say this has been the worst start to the fire season in 10 years - partly due to the 2012-2017 drought that killed off large amounts of vegetation.
In December, Governor Jerry Brown said devastating wildfires fuelled by climate change had become "the new normal", and that large fires "could happen every year or every few years".