The death toll from Southern California mudslides that swallowed dozens of homes and forced the closure of a major highway along the picturesque Santa Barbara County coast has risen to 20, with four other people still missing.

Emergency officials say the chances of finding more survivors in the ravaged landscape of hardened muck, boulders and other debris has waned considerably since heavy rains unleashed torrents of mud down hillsides before dawn last week.

Still, the 20 fatalities confirmed in and around the affluent community of Montecito, 137km north-west of Los Angeles in the coastal slopes adjacent to Santa Barbara, ranks as the greatest loss of life from a California mudslide in at least 13 years.

Ten people perished in January 2005 when a hillside saturated by weeks of torrential rains collapsed in the seaside hamlet of La Conchita, just 29km south-east of Montecito, burying more than a dozen homes in seconds.

Unlike the La Conchita tragedy, the stage was set for Montecito's slides by a massive wildfire last month – the largest on record in California – that stripped hillsides bare of any vegetation to hold soils in place after a day of drenching showers.

Another 900 emergency personnel arrived this weekend to join the relief effort conducted by more than 2100 personnel from local, state and federal agencies, including the US Coast Guard, the US Navy and the American Red Cross.

But authorities said the search-and-rescue mission had shifted into a "search-and-recovery" effort, reflecting the diminished likelihood of finding anyone else alive.

The destruction covered 78 sq km, leaving 65 single-family homes demolished and more than 450 others damaged. Nearly 30 commercial properties were damaged or destroyed, officials said.

The slides also forced a 16km stretch of one of California's most celebrated coastal roads, the heavily travelled Highway 101, to be closed indefinitely.