DARRINGTON, Washington (Reuters) - The death toll from a massive Washington state mudslide is expected to climb sharply over the next two days with 90 people still missing, authorities said on Thursday, even as they held out the faint hope of finding at least one more survivor.

The number of known dead stood at 25, five days after a rain-soaked hillside collapsed without warning, unleashing a wall of mud that engulfed dozens of homes in a river valley near the rural town of Oso, northeast of Seattle.

Only the first 16 victims recovered and examined by coroners have so far been officially counted as dead, and local fire district chief Travis Hots told a news conference that figure would soon spike upwards. Nine more bodies that have since been found have yet to be included in the official toll.

"In the next 24 to 48 hours, as the medical examiner's office catches up with the difficult work that they have to do, you're going to see these number increase substantially," he said.

Snohomish County officials said on Wednesday about 90 people were missing or unaccounted for, down from an earlier estimate that was nearly twice that number, and Hots said on Thursday that figure was holding.

Authorities said there was little chance of finding any more survivors in the square-mile heap of mud-caked debris and muck left by Saturday's landslide, and that the remains of some victims may never be recovered.

Still, Hots said a round-the-clock search effort by more than 200 people, who were painstakingly combing through the disaster site which included "clayballs the size of ambulances," would press on indefinitely.

"We're not changing the pace of this. And we're going to exhaust all options to try to find somebody alive," he said. "If we find just one more person that's alive, to me, that's worth it."

He said the forecast for Thursday called for rain, which may hamper search efforts.

'NEED A MIRACLE'

Community members from Oso and nearby towns assembled late Wednesday for a prayer vigil for the missing.

"We know, and most of us, I think, are accepting that many of our people are not going to make it," Megan Fanning, 41, said at the gathering in Darrington, not far from Oso.

"But please, we need a miracle. Just one. One little miracle would be wonderful," she said.

The community college student noted that the son of a close friend, 14-year-old Denver Harris, remained among the missing.

The known casualty count has pushed the Oso mudslide into the history books, said Josef Dufek, a professor at Georgia Institute of Technology who studies natural cataclysms.

He pointed to the 1980 eruption of Mount St. Helens in Washington, which killed 57 people, and a 1969 landslide in Nelson County, Virginia, which killed 150.

Emergency crews have used dogs, small cameras and sophisticated listening devices in the hunt for buried bodies, as other workers removed debris by hand.

The deeply traumatized people of the area rallied to comfort the bereaved and to support rescue crews.

Stores in nearby Arlington posted hand-painted signs calling for solidarity and donations, Boy Scouts collected food outside a market, and a bowling league offered to donate tournament prize money toward relief efforts.

"This is a very strong community. ... We all stick together," said 25-year-old Jamie Olsen, as her husband and about 40 people in Darrington sorted water, food, diapers and other supplies for families forced out of their homes.

Construction worker Steve Findley cooked breakfast for dozens of residents inside an Arlington middle school that the American Red Cross had transformed into a temporary shelter.

"All the people I know are gone," he said.

President Barack Obama has signed an emergency declaration ordering U.S. government assistance to supplement state and local relief efforts. A local disaster relief account had nearly $50,000 in it by Thursday.

Authorities have said the missing victims could also include people from outside the community about 55 miles (88 km) northeast of Seattle, such as construction workers or passing motorists.

Eight more people survived the slide but were injured, including a 22-week-old baby rescued with his mother. The baby was listed in critical condition but was improving. The mother and three other survivors remained hospitalized.

Jan McClelland, a volunteer firefighter from Darrington who was among the first to arrive at the scene, conceded it was possible some bodies may end up forever entombed at the site.

"I'm fearful we won't find everyone," she said. "That's the reality of it."