THE number of those believed missing following the deadly mudslide in Washington state has dropped to 30 after many people were found safe.

Authorities previously set the number of missing people at 90, but said they expected that figure to drop as they worked to find people and cross-referenced a list that included partial reports and duplicates.

Meanwhile, the confirmed death toll has risen by one, to 18. Finding and identifying all the victims could stretch on for a very long time and authorities have warned that not everyone may ultimately be accounted for after one of the deadliest landslides in US history.

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The search by heavy equipment, dogs and bare hands for victims from the slide was going "all the way to the dirt" as crews looked for anything to provide answers for family and friends a week after a small mountainside community was destroyed.

All work on the debris field halted briefly for a moment of silence on Saturday to honour those lost.

State governor Jay Inslee had asked people across Washington to pause at 10.37am, the time the huge slide struck on March 22, destroying a neighbourhood in the community of Oso, north of Seattle.

The huge wall of earth that crashed into the collection of homes followed weeks of heavy rain.

Previous slides triggered by storms included one that killed 150 people in Virginia in the wake of Hurricane Camille in 1969 and another that killed 129 when rain from Tropical Storm Isabel loosened tons of mud that buried the Puerto Rican community of Mameyes in 1985. A dam in San Francisquito Canyon, California, collapsed in 1928, causing an abutment to give way and killing 500 people, according to data from the US Geological Survey.