The US town devastated by a monster mudslide that has claimed up to 24 lives with 170 still missing so far was warned four years ago of the risk, but residents felt safe anyway, officials say.

As workers probed the disaster site for missing residents of Oso, Washington, the little town buried last weekend under 2.5 square kilometres of earth and timber, emergency preparedness officials defended their own performance.

Searchers are facing more bad weather as they are hampered by quicksand like mud, calling mobile phones of the missing in the hope they will be found.

A 2010 report commissioned after a major landslide in 2006 pinpointed the hillside overlooking Oso as among the most dangerous in terms of possible collapse.

But officials – who believe Saturday’s landslide may have been triggered by a small earthquake – say the local community was aware of the dangers.

Townspeople “knew the risk, but they felt safe in the small events,” John Pennington, head of the Snohomish County Emergency Management Department, told reporters.

“This wasn’t a small event. It was large, it was very catastrophic,” he added, fighting back tears at one point.

Some 200 rescuers digging through the debris recovered two more bodies on Tuesday and believe they located eight more, officials said.

On Wednesday, they said the official confirmed death toll remained at 16, as the other bodies found had not been extricated.

The number of reports of people unaccounted for remains at 176, although that could include double-counting and people who may turn up elsewhere, so likely does not represent how many more fatalities may be found.

“I’m very confident that that number will drop in the end,” Pennington said.

Forty-nine dwellings in the rural town were hit by the 2.5 square kilometre wall of mud, rocks and trees, which also destroyed part of a highway some 95 kilometres northeast of Seattle.

Officials say the massive emergency operation is still aimed at rescuing survivors as well as recovering bodies, even though the chances of finding people alive diminish with each day.

The National Guard joined local and state emergency workers after President Barack Obama declared a state of emergency for Washington state.

The Federal Emergency Management Agency (FEMA) will help “save lives and to protect property and public health and safety, and to lessen or avert the threat of a catastrophe in Snohomish County,” said the White House.

One volunteer firefighter described the landslide as “quicksand, where you could easily sink up to your armpits”.

There were signs of structures peeking through a dense layer of fallen timber and earth, he said, adding that debris had blocked the Stillaguamish river, causing floods in the Darrington area.