Two young children and their 70-year-old great-grandmother have died in the wildfire that is sweeping northern California with devastating speed.

The fire has forced 38,000 residents to flee their homes, razed 500 buildings and killed five people -- three civilians and two firefighters.

President Donald Trump declared a state of emergency on Saturday and a tearful Sherry Bledsoe confirmed the deaths of her grandmother, Melody Bledsoe, and her children, James Roberts, 5, and Emily Roberts, 4 in the city of Redding.

Don Ray Smith, a bulldozer operator who died working to contain the fire was identified as an 81-year-old man from a small community east of Sacramento.

The so-called Carr Fire started burning on Monday. It exploded on Thursday, jumped the Sacramento River and entered the Redding city limits.

Authorities previously identified another firefighter fatality as Redding Fire Department Inspector Jeremy Stoke, who was killed on the job Thursday night.

The Bledsoes lived near the Keswick Estates neighbourhood, and their home, like many around it, was reduced to ashes. The tragedy leaves Ed Bledsoe without his wife and the great-grandchildren he doted on.

He had headed out for supplies on Thursday thinking the flames were far away, but while shopping he received a desperate call from his great-grandson. The boy said he had to come back to the home because the flames were closing in.

The family believed the area was not in imminent danger and Melody Bledsoe had no car.

Ed Bledsoe rushed home, but was turned back by police. The fire was raging and there were walls of flames.

There are currently 89 large wildfires blazing across 14 US states, mostly in the West, according to the National Interagency Fire Center.

So far this year, wildfires have scorched almost 1.7 million hectares across the US, above the 1.5 million-hectare average for the same period over the last decade.