Three weeks after flames incinerated most of a Northern California town in the deadliest US wildfire in a century, the search for more human remains has officially ended with at least 88 people confirmed dead and nearly 200 still listed as missing.

Butte County Sheriff Kory Honea said he was optimistic that some who remain unaccounted for will turn up alive, but he also left open the possibility that "bones or bone fragments" of additional victims could turn up as evacuation zones are reopened to civilians.

With the fire reduced to embers, the National Weather Service on Thursday issued a flash-flood warning for the burn zone as showers and thunderstorms heightened the risk of heavy runoff in areas stripped of vegetation by the fire.

At a news conference on Wednesday night Honea said search and recovery teams had finished combing through the ruins of approximately 18,000 homes and other buildings leveled by the Camp Fire, which ranks as the most destructive in state history.

The bulk of the devastation occurred in and around the hamlet of Paradise, a town once home to nearly 27,000 people, many of them retirees, in the Sierra foothills about 175 miles (280 km) north of San Francisco.

More than 1,000 personnel, including cadaver dog teams, forensic anthropologists, coroners and National Guard troops from five states, took part in the grim effort.

"I believe that we have done our due diligence with regard to searching for human remains. My sincere hope is that no additional human remains will be located," Honea told reporters in the nearby town of Chico.

Asked directly whether authorities had ceased actively searching burned structures, the sheriff answered yes.

The current death toll of 88 already stands as the greatest loss of life on record from a single wildfire in California and the most from a wildfire anywhere in the United States dating back to Minnesota's 1918 Cloquet Fire, which killed as many as 1,000 people. The Camp Fire also exceeds the 87 lives lost in the Big Burn firestorm that swept the Northern Rockies in 1910.

Authorities attribute the Camp Fire's high casualty count in large part to the tremendous speed with which flames raced through Paradise with little advance warning, driven by howling winds and fueled by drought-desiccated scrub and trees.

The remains of many victims were found in the ashen rubble of homes, others inside or near the burned-out wreckage of vehicles.

The cause of the blaze, which was fully contained earlier this week, remained under investigation. But PG&E Corp reported equipment problems near the origin of the fire around the time it began on Nov. 8.

The official roster of people unaccounted for has fluctuated widely from day to day, but as of Wednesday night the list was winnowed to 196 names, down from a peak of 1,200-plus over a week ago.

The sheriff said the list had since been scrubbed of all duplicate names and that investigators had caught up with a backlog of unprocessed missing-persons reports.