Rescue workers Sunday were searching through mud and debris to locate at least 18 people still unaccounted for in a mudslide that so far has claimed three lives.The disaster in the north-west US state of Washington occurred Saturday, when the side of a mountain in Snohomish County suddenly slammed into the homes, highway and river below it.At least 11 people were injured, including a six-month-old baby listed in critical condition, according to The Seattle Times newspaper.Fire Chief Travis Hots, of the Snohomish County Fire District 21, said Sunday at a press conference that the 18 reported missing were residents of neighbourhoods that were directly in the path of the mudslide, according to the Herald newspaper of Everett, Washington.It was possible that people driving on Highway 530 have also been buried or washed away, Hots said.The deep, unstable mud has made it extremely unsafe for rescuers, who had to give up a search Saturday night to find someone yelling for help from a buried house, officials said.Officials urged residents downstream along the Stillaguamish River to evacuate, out of concern that the water building up behind the unstable mud dam blocking the river could force through in a sudden flood.About 100 rescuers were on the ground or in the air, Hots said.Geologists were also surveying the area from the air to see what could be done to stabilize things to protect rescue workers.