Rescue crews have expanded their search for five missing people as the death toll rose to 18 from Southern California mudslides that damaged hundreds of buildings and caked highways with sludge.

About 1,250 emergency workers raced against the clock to find survivors with drones, heavy equipment and sniffer dogs in the rescue and clean-up efforts, the California Governor’s Office of Emergency Services said.

The latest victim, 87-year-old Joseph Bleckel, was found in his Montecito home on Friday (local time), Santa Barbara County Sheriff Bill Brown said, adding that the remaining missing were between the ages of two and 30.

“We’ve got a window that’s closing, but we’re still very optimistic. There’s been plenty of cases where they’ve found people a week after,” Santa Barbara County Fire Department spokesman Mike Eliason said on a muddy Montecito street especially hard-hit by the mudslide.

Secondary searches of damaged structures were underway.

The number of missing people fluctuated as some were located. On Thursday night 43 people were unaccounted for.

Residents in some areas were subject to a new mandatory evacuation on Friday, emergency officials said, adding the unstable environment remained a threat.

Triggered by heavy rains, the massive slides struck before dawn on Tuesday.

Walls of mud and debris cascaded down hillsides stripped of trees and shrubs by last month’s wildfires, including the Thomas Fire, the largest blaze in the state’s history.

A search and rescue dog looks for victims.

Excavators carrying rescuers in their buckets ploughed through mud-coated roads in search of the missing after some areas were buried in as much as 4.6 metres of mud, emergency officials said.

“It is heavy. It’s wet. It just exhausts the crews out there,” Sacramento Fire Department Captain Pat Costamagna said in a social media video from the Governor’s emergency management office.

County officials have already ordered residents in most of the south-eastern corner of Montecito, an unincorporated community east of the city of Santa Barbara, to leave their homes for what they said was likely to be one or two weeks to aid the search and recovery efforts.

In one of the worst-hit areas of Montecito, mud blew through doors and windows, filling the interiors of houses with muck and debris.

The walls at one end of a home had disappeared, leaving its roof hanging precariously.

Downed power lines wrapped around trees at one property, while elsewhere the lines dropped almost to the ground. Elsewhere, cars were perched on mounds of earth and garage doors had caved in.

The cause of death for most of the victims will be listed as multiple traumatic injuries resulting from flash floods with mudslides, the Santa Barbara Sheriff’s office said. The dead ranged in age from three to 89.

Residents of the mudslide-hit area were assessing their damaged homes.

Garret Speirs, a 54-year-old artist who has been living in Montecito for 20 years, was grateful his property had survived.

“We have a yard to redo and hopefully our insurance will help out with that, but the people across from me, newer homes, gone,” he said.

“Everybody down below gone, two girls gone,” Mr Speirs said. “Two sixth-graders in the school our kids went to.”