Seattle and King County officials told reporters that that it had documented at least 14 cases, leading to five fatalities.
A short time later, Snohomish County Health District spokeswoman Heather Thomas confirmed a coronavirus-related death was of a man from her jurisdiction, bringing the state's death toll to six.
"We expect the number of cases will continue to increase in the coming days and weeks, and we are taking this situation extremely seriously," said Dr. Jeff Duchin, chief health officer for the Seattle and King County Public Health agency.
"The risk for all of us of becoming infected will be increasing. And although most of the cases will be mild or moderate, the infection can cause serious illness and there's a potential for many people to become ill at the same time."
The five who died in King County were two men, in their 50s and 70s, and three woman, one in her 70s and two in their 80s, officials said. The Snohomish County death was a man in his 40s.
"As you are undoubtedly observing, this is a complex and unprecedented challenge, locally, nationally and globally," Duchin said.
Patty Hayes, a registered nurse and director of public health for Seattle and King County, urged citizens to refrain from buying up masks and to leave that supply for others.
"We need assets for our healthcare front-line workers and also for those who may be needing them and are not feeling well so they want to protect themselves or those who culturally use them," she said.
"Masks are generally not recommended for healthy people to prevent infection (but) they are so important for these other uses."
King County Executive Dow Constantine said his office is on verge of buying a motel where quarantined patients can be housed.
"We have moved to a new stage in the fight to contain, mitigate and manage this outbreak," he said. "We continue to plan for all contingencies."
Nearly a dozen schools were closed in western Washington on Monday, over fears of the virus.