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At least 18 people are unaccounted for after a landslide in rural Washington state, the fire chief said at a news conference Sunday.

The landslide on Saturday, in an area north of Seattle, encompassed about one square mile and resulted in three deaths, Snohomish County Fire District 21 Chief Travis Hots said. Seven people have been taken to the hospital.

The landslide was caused by groundwater saturation tied to heavy rainfall in the area over the past month, authorities said.

While there’s a tremendous effort to rescue people who may be trapped, Hots told reporters that the rescue operation must be focused Sunday on keeping responders safe because the area is highly unstable.

The mud flow is like quicksand, he said. Responders suspect there are survivors, but it is “far too dangerous” to get rescuers to those people. The landslide is 15 feet deep in some places, emergency officials said.

On Saturday rescuers dug through the rubble while survivors were crying for help underneath the debris, Hots confirmed. The fire chief that rescuers heard voices around 11:30 p.m. and considered trying to reach the possible survivor or survivors, but “the mud was too thick and deep,” Hots said, and rescuers had to back off.

For the time being, helicopters are surveying the area so that an assessment can be made as to when and how it will be safe for responders to attempt to help people.

Sunday afternoon, Washington Gov. Jay Inslee, joined by other state and local leaders at a news conference, called the rescue operation “aggressive.”

“Every human endeavor … is being explored here to rescue and find their loved ones,” he said.

Inslee added that some rescuers had gotten “caught…up to their armpits” in the slide and “had to be dragged out by ropes.”

John Lovick, a county executive, addressed a reporter’s question about whether voices were still being heard.

“We were told that there were noises in that area,” Lovick said, stressing that fire chief Hots had decided that it was “too risky” to place rescuers in that area. “We are not hearing any reports of people hearing voices today or after last night.”

Inslee said he plans to talk with federal officials about Federal Emergency Management Agency relief, adding that he’d never before seen firsthand the kind of devastation he witnessed by surveying the area by helicopter.

As of Sunday morning, Harborview Medical Center spokeswoman Susan Gregg told CNN it is treating five patients: a 25-year-old woman in satisfactory condition; a 6-month-old boy in critical condition in the intensive care unit; a 37-year-old man in serious condition in the intensive care unit; a 58-year-old man in serious condition in the intensive care unit; and an 81-year-old man in critical condition in the intensive care unit.

At least six houses were destroyed in the landslide, and as many as 16 were damaged, the Snohomish County Sheriff’s Office reported earlier Sunday.

The landslide affected the towns of Oso, a remote community of about 180, and Darrington, a town of about 1,350 people. The landslide cut off State Road 530 to Darrington. Part of the Stillaguamish River also was blocked, and residents were warned of possible flooding both upstream and downstream of the collapse.

The Washington State Patrol provided photos that showed floodwaters and sprawling debris covering a rural patch of the road, framed by woodlands and snow-capped mountains.

The first reports of the landslide came in around 10:45 a.m. Saturday (1:45 p.m. ET), the sheriff’s office said. CNN first learned of the landslide via Twitter.

The county later said “we strongly recommend” that those living in the north fork of the Stillaguamish River flood plain, from Oso to Stanwood, “evacuate your home immediately.”

“We are working on establishing shelters for those who have nowhere to go,” county spokeswoman Rebecca Hover said in a statement. “Until then, people should get to higher ground as soon as possible.”

A number of agencies have responded, including the state transportation and emergency management departments, the U.S. Navy and fire departments across Snohomish County.

Upon arriving at the scene, firefighters and state troopers heard calls for help, trooper Mark Francis said.

The Snohomish sheriff warned people to stay clear of trestles or bridges or anywhere near the Stillaguamish River downstream of the slide.

“Water could break through at any moment,” the sheriff’s office tweeted.

By Ashley Fantz, Joe Sutton and Janet DiGiacomo

CNN’s Michael Martinez and AnneClaire Stapleton contributed to this report.