ARLINGTON (AFP) - The US town devastated by a monster mudslide that has claimed up to 24 lives so far was warned four years ago of the risk but residents felt safe anyway, officials said on Wednesday, March 26, 2014.

As workers probed the disaster site for missing residents of Oso, Washington - the small town buried on Saturday under 2.5 sq km of earth and timber - emergency preparedness officials defended their performance.

They pointed to 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.

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

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

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 missing people has fallen to 90, from an earlier figure of 176, officials said.

However, Washington Governor Jay Inslee said he expected the death toll to rise significantly.

"I don't think anyone can reach any other conclusion," he said when asked about that prospect on CNN.

"The force of this landslide just defies imagination ... It was 100 per cent devastation within the contours of the slide," he added.

A total of 49 dwellings in the rural town were hit by the 2.5 sq km wall of mud, rocks and trees, which also destroyed part of a highway some 95km north-east 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 the 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," the White House said.

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.

"The rising river and floodwaters are making the slide areas nearest the banks increasingly unstable," the emergency worker, who did not give his name because he was not authorised to speak to the media, told AFP.

Some believe that a small 1.1 magnitude earthquake in the area may have triggered last Saturday's landslide. The possibility was mentioned in a briefing earlier in the week.

Others said recent rain had saturated the hillside and so increased the risk of a slide, while the Seattle Times said long-term logging allowed on the plateau above may have made the area more unstable.

Governor Inslee rebuffed questions about whether authorities could have done more to warn people of the risk in recent days.

"We will get to the bottom of that question when we have time to thoroughly evaluate it," he told CNN.

Holding out hope for survivors, he added: "I am dedicated 24 hours a day right now to rescuing these people."