Police in the Italian-speaking Alpine canton said in a statement that the landslide had occurred on higher ground in the remote Maggia valley early on Sunday.
A bridge downstream of the disaster area was submerged by water, complicating rescue efforts, they added.
Two bodies recovered in the Fontana area of the valley were currently being identified.
Authorities have evacuated campsites along the Maggia river, while telecommunications and power supplies have also been disrupted, the police statement said.
SD/PR