OLYMPIA (REUTERS) - The death toll rose to four and 18 people were missing on Sunday from a landslide in Washington state that buried homes and cars under mud and debris up to 5m deep, authorities said.

Eight people were also injured when rain-soaked embankments along State Route 530 near Oso, Washington, about 90km northeast of Seattle, gave way on Saturday morning and washed out at least six homes.

Snohomish County Fire District 21 Chief Travis Hots said rescuers had found a fourth victim, but they had yet to recover the body.

"I am disappointed to tell you that after searching a very large area of that debris field on foot, we didn't find anybody alive. There was no sign of life," Hots told a televised news conference.

The tally of missing was likely to grow, he said. The rescue mission will halt at dusk because of treacherous, quicksand-like conditions, and will resume at dawn on Monday, Hots said.

Washington Governor Jay Inslee flew over the area on Sunday.

He told the news conference, "I have a sense that we're going to have some hard news here."

The slide occurred in the foothills of the Cascade Mountains along the Stillaguamish River. The slide covered about 2.6 sq km and left mud, rock and debris up to 5m deep in some places.

It blocked the flow of the river, creating floods and a backup of water behind a natural dam of mud and debris, but the threat to people downriver had begun to ease, Inslee said.

The highway was closed in both directions, with no timeline for it to be reopened, he said.

The Snohomish County sheriff's office has asked people affected by the slide to report to the Red Cross so an accurate count can be made of the missing.

Washington state Lieutenant Governor Brad Owen on Saturday evening declared a state of emergency in Snohomish County.