The death toll from the monster US landslide is expected to rise "substantially," somber officials warned Thursday as the scale of the disaster becomes clear with 90 still missing five days on.

The confirmed number of dead stands at 16 with at least eight more bodies believed located after the monster mudslide crashed onto the town of Oso.

But Snohomish County District 21 Fire Chief Travis Hots said bodies were not being included in the official death toll until they had been recovered and "medical officers have done their difficult work."

- 'Numbers to increase substantially' -

"You're going to see these numbers increase substantially" in the next day or two, he told reporters.

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

More than 200 rescue workers have been working in grim conditions for the last five days. A few survivors were found immediately after the mudslide, but none since Saturday.

Rescue volunteer Kraig Wenrick said the task was both physically and mentally difficult, as they dug "right into the heart" of the landslide mass.

"That's what we've got to prepare our minds for... finding bodies. If they're there," he told AFP in Darrington, the town on the east side of the slide.

"Ultimately we'd like to find survivors," he said, but added: "It's getting hard to know."

Emergency officials have come in for criticism as the rescue operation drags on, with some locals frustrated that they have not been allowed to help in the search.

Authorities also defended themselves against suggestions they could have given more warning about the landslide. A 2010 report following a 2006 mudslide identified the Oso region as facing a major risk.

The cause of the landslide remains unclear. Some say recent rain increased the risk of a slide on the saturated hillside, while The Seattle Times said long-term logging allowed on the plateau above may have made the area more unstable.

The US Geological Survey dismissed a suggestion, mentioned by emergency management chief John Pennington earlier in the week, that a small earthquake could have triggered it.

"There are no earthquakes or other local seismic events on the records around the time of the landslide," said a USGS statement.

Fire chief Hots said workers have not given up hope of finding survivors, even if the chances diminish by the day.

"If we find one more person alive, to me that's worth it," he said.

- Clay balls 'size of ambulances' -

But the scale of the disaster was staggering, when viewed up close.

"You just can't fathom what we're up against... until you get out there and see the lay of the land," Hots said.

"You've got clay balls the size of ambulances that have rolled off that hill and smashed everything that they've come into contact with."

He described workers recovering a victim Wednesday from a car that had been swept 200 feet (60 meters) from the highway, in a process that took several hours.

"What happens is, when the person is removed, things kinda get somber out there. It gets really quiet for a few minutes. You can almost hear a pin drop," he said.

"You see seasoned veterans in this business, they start to tear up, their eyes get glossy. It's kinda their way of paying respects to these people's loved ones."