Three people have been confirmed dead and 18 missing after a landslide in northwestern Washington state in the US.   

“We've got this huge square mile mud flow up there, which is basically like quick sand. It's extremely fluid, it's moving. We suspect there are people out there but it is far too dangerous to get responders out there on the mud flow,” Snohomish County's Fire District 21 Chief Travis Hots said at a news conference.

Mr Hots said seven people were transported to area hospitals with injuries.

Snohomish County's Director of Department of Emergency Management John Pennington said the mud is flowing from a river and dumping into a lake, which will eventually fill up.

An eyewitness to the mudslide, Paulo Falcao, took footage on his cell phone of debris covered in mud, emergency workers and what appears to be remains of a building on the road.

The slide happened as rain-soaked embankments along State Route 530 near Oso, Washington, about 88km northeast of Seattle close to the Canadian border, gave way at about 10.40am local time on Saturday.

The mudslide caused blockage of the Stillaguamishi River, adding flooding to the mud problem.