Two children and their great-grandmother are among the six killed in a Northern California wildfire as fire crews battled on to quell flames that have destroyed entire neighbourhoods.

More than 38,000 people remained under evacuation orders on Sunday in and around the city of Redding, about 257 kilometres north of the state capital Sacramento, from a blaze that has destroyed more than 500 buildings and continued to rage largely unchecked into a seventh day.

The Carr Fire, the deadliest and most destructive of nearly 90 wildfires burning from Texas to Oregon, has charred 36,095 hectares of drought-parched vegetation since erupting last Monday.

More than 5000 structures were threatened by the fire, officials said. The flames destroyed 517 structures and damaged 135.

The weather on Sunday is expected to offer no relief for firefighters as it will hit almost 38 degrees, with low humidity and gusty winds, the National Weather Service said.

An army of 3500 firefighting personnel and a squadron of 17 water-dropping helicopters had managed to carve buffer lines around just 5 per cent of the fire’s perimeter as of Sunday.

Fire officials say the erratic behaviour of the blaze, stoked by high winds and soaring temperatures, has complicated efforts to contain the conflagration.

A firefighter pauses for a moment during firefighting efforts. Photo: Getty

President Donald Trump on Saturday declared the fire an emergency, authorising federal funds for disaster relief efforts.

At the height of its fury on Thursday night, the fire was whipped into a storm-like frenzy by gale-force winds that drove flames across the Sacramento River into the western end of Redding, as thousands of residents fled for their lives in a chaotic evacuation.

The nearby town of Keswick, with a population of about 450, was reduced to cinders, and two firefighters were killed.

On Saturday, Redding police said they were searching for 17 people still unaccounted for two days afterward.

Redding Police Sergeant Todd Cogle confirmed that the three bodies discovered at a fire-ravaged home on the outskirts of Redding were two children and their great-grandmother.

Shasta County Sheriff Tom Bosenko said they had not yet been positively identified but were believed to be victims identified by relatives on Facebook and in news media reports as James Roberts, 5, his sister Emily, 4, and Melody Bledsoe, 70.

Ms Bledsoe’s granddaughter, Amanda Woodley, said on Facebook that the elderly woman desperately put a wet blanket over the children as their home burned.

“Grandma did everything she could to save them. She was hovered over them both with a wet blanket,” Ms Woodley said in a post.

-AAP