Rescue crews in Southern California have expanded their search for five missing people as the death toll following heavy mudslides rose to 18.

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, an 87-year-old man, was found in his Montecito home, Santa Barbara County Sheriff Bill Brown said. He added that the remaining missing people were between the ages of 2 and 30.

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

Officials said secondary searches of damaged structures were under way.

The number of missing has fluctuated as people were located, said the Santa Barbara Sheriff's Office. Authorities said on Thursday night that 43 people were unaccounted for.

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

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

In addition to the dead and injured, 65 houses have been destroyed while another 462 sustained damage.

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.