The confirmed death toll from a huge US landslide rose from 18 to 21 on Sunday, while four more bodies were located but not counted in the official tally, authorities said.

Thirty people remain missing after a rain-soaked wall of mud, trees and debris crashed down onto the edge of the town of Oso, in Washington state, on March 22.

A few survivors were found immediately after the mudslide, but none since the day of the catastrophe.

The number confirmed dead has hit 21, Snohomish County officials said, but is set to rise again once four more bodies that have been located are recovered and formally identified by the county's Medical Examiner's Office.

Dozens of dwellings in the rural town, which is 60 miles (95 kilometers) from Seattle, were wiped out in the landslide, which was so ferocious that it also destroyed part of a highway.

Rescue workers have been working in tough conditions for more than a week -- the sludge was so thick in places that rescuers themselves have had to be saved from being swallowed up.

March saw far-above average rainfall for both Arlington and Darrington, the towns either side of Oso that have been cut off by the mudslide, according to County Emergency Management Department program manager Jason Biermann, citing official meteorological figures.