The search for survivors of the deadly mudslides in parts of California’s Santa Barbara County is continuing even as hopes of finding anyone alive dwindled.

“We’re still in rescue mode and we still hope to find someone alive, although the chances of that are becoming slim,” said Justin Cooper, a spokesperson for the multi-agency response team.

The death toll rose to 20 on Sunday with four people still missing, Mr Cooper said.

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.

The ramped-up rescue effort is in response to urgent requests for additional manpower made earlier in the week.

Heavy rains on Tuesday soaked the area near Montecito, north of Los Angeles, where vegetation had been stripped away by the largest wildfire in California’s history last month.

More than 2000 personnel are continuing the grim search for survivors. Photo: Getty

Sodden hillsides gave way, unleashing a torrent of mud, water, uprooted trees and boulders on to the valley below, killing 20 people aged 3 to 89.

The destruction covered 78 square km, according to the California Department of Forestry and Fire Protection, and forced the partial closure of one of California’s most celebrated coastal roads, the heavily used Highway 101.

Officials ordered residents in most of the southeastern corner of Montecito, which is east of Santa Barbara, to leave their homes for what was likely to be one or two weeks.

Many fled to nearby Carpinteria, where local resident Tessa Nash said they were communicating via a Facebook page called Carpinteria Swap, which is usually focused on buying and selling second-hand goods.

In the past few days, Ms Nash said, it has been carrying information about community-led blood drives and transportation tips.

“We’re really joined together,” she said. “We’re affected here in Carpinteria in the sense that we’re taking these people in and a lot of people are out of work because they can’t travel. It’s a trickle-down effect.”