The number of people missing after a devastating US landslide has fallen to 90, officials said Wednesday while defending themselves over the disaster which has claimed as many as 24 lives so far.

The confirmed death toll remained at 16 with 8 more bodies believed located, as an army of rescuers struggled in grim conditions amid the vast blanket of mud, timber and debris left by last weekend's mudslide.

But Washington state governor Jay Inslee warned that he expected the death toll to rise significantly, as workers dig into the mile square (2.5 sq km) block of hillside which crashed down on the town of Oso.

- 'Landslide just defies imagination' -

"I don't think anyone can reach any other conclusion," he told CNN, when asked about the prospect of many more bodies being found.

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

At an evening briefing, Snohomish County emergency management chief John Pennington said 140 people previously unaccounted for have been found, while the status of another 35 was unknown.

The updated figures -- down from 176 listed unaccounted for since Monday -- came as managers defended their record, noting that experts had warned of the landslide risk four years ago.

- 'People knew the risks'

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," Pennington told reporters.

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

A total of 49 dwellings in the rural town were hit by the one square mile (2.5 square kilometer) wall of mud, rocks and trees, which also destroyed part of a highway some 60 miles (95 kilometers) 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 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.

- Locals frustrated -

While the official recovery operation cranked into higher gear with 200 workers attacking the landslide area from both sides, there were signs of frustration from some locals.

Speaking in Darrington, one of the nearest towns, resident Kraig Wenrick said people in the local community were being told they could not help despite knowing the local area intimately.

"It's tearing it apart," he told AFP. "Everybody's upset about it because they need to just get down there and let us do our job. We know this community, we know the housing down there we know where everybody lived.

"We know this area better than anybody, and to have people come in here who don't know nothing, and try to tell us how to do this, just isn't acceptable," he said.

- Earthquake, rain, logging? -

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."