LOS ANGELES (AFP) - More than 100 people are still unaccounted for after a devastating landslide in the north-western United States state of Washington, while eight people are so far confirmed dead, officials said on Monday.

The number reported missing or unaccounted for rose dramatically from 18 to 108 after the massive landslide slammed "like a freight train" into a mountainside in Snohomish County.

"We're still in rescue mode here, but 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.

"To date there are 108 reports of names of individuals who are either unaccounted for or missing," he said. "This doesn't mean that there are 108 injuries, or 108 fatalities, it's 108 reports," he told reporters.

"It was Saturday, and it was probably a higher number than you would see during a weekday," he said.

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

The field of rubble is about 2.4km across and some 4m to 6m deep in areas, The Seattle Times reported.

Rescuers reported hearing voices calling for help on Saturday, but Mr 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.

Washington state Governor Jay Inslee, who declared a state of emergency for the area, told reporters there is "a full-scale, 100 per cent, 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.

People injured in the landslide include a six-month old infant and an 81-year-old man, both hospitalised in critical condition at a Seattle hospital, local media said.

"It sounded like a freight train," landslide witness Dan Young told Komo4News. "In just 35 to 45 seconds it was over."

Mr Young's home survived but is flooded. "It's much worse than everyone's been saying," a firefighter who did not want to be named told The Seattle Times.

"The slide is about a mile wide. Entire neighborhoods are just gone. 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.

Ms 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.