The number of people missing after a landslide sent a wall of mud crashing into dozens of rural Washington state homes has dropped to 90.

It comes as officials reported finding more bodies but acknowledged some victims' remains may never be recovered.

Four days after a rain-soaked hillside collapsed near the tiny town of Oso, a fire official said the death toll had risen to 25 people, including nine whose bodies remained in the debris.

Officials had earlier said additional remains had been found in the devastation zone about 90km northeast of Seattle yesterday.

However, they declined to say how many until they had been removed and sent to a medical examiner's office.

As hope faded that any survivors might be plucked from the muck and debris, residents of the stricken community and nearby towns braced for an expected rise in the casualty count.

"My son's best friend is out there, missing," said John Pugh, 47, a National Guardsman who lives in the neighbouring village of Darrington.

"My daughter's maid-of-honour's parents are missing. It's raw. And it will be for a long time," he said.

Asked whether he expected the death toll to rise significantly, Governor Jay Inslee said: "Yes, I don't think anyone can reach any other conclusion."

"It's been very sad that we have not been able to find anyone living now for probably 36 or 48 hours.

"The most discouraging thing is we were hopeful that we would find folks who might be protected by a car or a structure, but the force of this landslide just defies imagination."

About 180 people were known to have lived in the path of the landslide, although not all would have been home at the time of the slide on Saturday, according to Snohomish County's emergency management director John Pennington.

Authorities who whittled down a list of missing from about 176 people to 90 have said the victims could also include people from outside the community, who were there at the time of the mudslide.

The fate of up to roughly 35 more people not counted officially among the missing remained uncertain, Mr Pennington said.

Yesterday evening, Brian McMahan, Assistant Fire Chief of the community of Mukilteo, told 250 people at a community meeting in nearby Darrington that one additional body had been found that day, bringing the known total to 25.