By Kevin Murphy
March 23 (Reuters) - Eighteen people remain unaccounted for following a landslide in northwest Washington state that has already claimed three lives and injured at least eight more, officials said on Sunday.
The landslide destroyed at least six homes as it tumbled across a state highway on Saturday, leaving a layer of mud, rock and debris 15 to 20 feet deep in some places, said Shari Ireton, spokeswoman for the Snohomish County sheriff's office.
Ireton said it is unclear if the missing people are trapped or have simply not reported their whereabouts. The 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.
The slide happened as rain-soaked embankments along State Route 530 near Oso, Washington, north of Seattle gave way. The area covered by the slide is estimated at one-half mile wide, Ireton said.
The slide occurred in the foothills of the Cascade Mountains along the Stillaguamish River. The slide has blocked flow of the river, creating floods and a backup of water behind a natural dam of mud and debris that could give way, Ireton said.
"We have a serious potential situation, should that be breached," Ireton said.
Helicopters, earthmovers, fire rescue vehicles and other equipment are being used to search for and remove any victims still in the rubble, officials said. The highway was closed in both directions. (Reporting By Kevin Murphy; Editing by Edith Honan and Jan Paschal)