More than 100 people remained unaccounted for Monday after a devastating landslide in the US state of Washington which killed at least eight and sounded "like a small earthquake," witnesses said.

The number rose dramatically from 18 to 108 after the massive landslide slammed into a mountainside town in Snohomish County north of Seattle on Saturday.

"The situation is very grim," said Snohomish County fire district chief Travis Hots. "We're holding out hope that we'll find people that are still alive, but we haven't found anyone alive since Saturday."

Emergency management chief John Pennington stressed that 108 is the number of reported missing or unaccounted for, not necessarily actually missing after the disaster on Saturday.

But he said there were a total of 49 dwellings of various types in the area hit by the devastating landslide, and that there were likely to have been more people at home on a Saturday than during the week.

- Voices heard, then silence -

The wall of mud, rocks and trees smashed into the rural town of Oso, northeast of Seattle in the northwestern US state, destroying houses and part of a highway.

Some 100 emergency workers were searching for survivors in the field of mud and rubble about 2.4 kilometers (1.5 miles) across and some four to six meters (13-20 feet) deep in areas.

Mini hovercraft were used to skate across the vast mudslide's surface, while tracker dogs and helicopters were also being used.

Rescuers reported hearing voices calling for help on Saturday, but Hots said they "didn't see or hear any signs of life" on Sunday.

Among the missing was a four-month-old baby and her grandmother, local media reported.

Oso resident Doug Dix, whose house was a couple of hundred yards from the slide, said he was working in his barn when he heard a huge rumbling noise.

- 'Like a small earthquake' -

"My first impression was I thought we were having a small earthquake. The barn was vibrating," the semi-retired wildlife toxicologist told AFP.

"Then I went outside and it sounded to me like one of those twin-prop helicopters coming down ... It was unbelievably noisy." The noise went on for about a minute. "I was looking up in the air trying to find a plane crash," he said.

Dix said he "would be surprised" if the figure of over 100 unaccounted for remained that high. "They were saying initially 18 people missing, which seems more reasonable," he said.

Shari Ireton of the Snohomish County Sheriff's Office said the 108 figure could include double counting, as it was the result of combining a number of lists of people missing, not always with full names.

"Some of those could be overlapped," she told a lunchtime briefing, adding that the death toll of eight remained the same.

Washington state Governor Jay Inslee, who declared a state of emergency for the area, told reporters there is "a full-scale, 100-percent, aggressive rescue effort" going on, adding that helicopters, hovercrafts and rescue personnel had rushed to the scene.

The muddy area was so unstable that some rescue workers "went in and got caught literally up to their armpits" and had to be pulled out themselves, Inslee said.

"The slide is about a mile wide. Entire neighborhoods are just gone," a firefighter who did not want to be named told The Seattle Times. "When the slide hit the (Stillaguamish) river, it was like a tsunami."

Rain has been especially heavy in the Cascade Mountains region in the past weeks. The forecast is for more downpours throughout the week.

Patty Murray, who represents Washington in the US Senate, gave assurances that federal resources would be made available, as she offered thanks to rescue workers and her prayers to the families of the ravaged community.