The death toll from the mudslide last month in the US state of Washington has reached 39.

All of the dead have been identified, four people remain on the missing list, Snohomish County officials said on Friday.

The search area has narrowed, but crews continued to look for human remains, according to a statement by Snohomish county officials.

Officials urged people to stay away from the mudslide area, saying it is still too dangerous to visit.

The March 22 mudslide completely buried the community of Oso, 100 kilometres north-east of Seattle.

The landslide followed a month of record-breaking rain in Washington, which borders Canada on the Pacific Coast.

No survivors have been located in the huge debris field since the immediate aftermath of the disaster.

US President Barack Obama will visit the area on Tuesday and will meet with families, first responders and recovery workers.