The villages of Bondo, Promontogno, Sottoponte and Spino have been evacuated as a precaution. "The missing persons' relatives don't know exactly where they were", stated local police official Andrea Mittner during a press conference held near the town. According to local residents, there is a missing climbers.
Images showed a broad swathe of farmland covered in a grey, moving mass, with mud partially engulfing some buildings.
"It was awful", Elisa Nunzi, 27, who witnessed the landslide from her home in a neighbouring higher-altitude village, told Swiss daily Blick.
"Many have spent the night in the Home of Spino, a village just in front of Bondo", she added.
"You could not see anything for an hour".
Helicopters and rescue services were leading the search for the missing on Thursday.
Initial estimates found that up to 4 million cubic meters (140 million cubic feet) of rubble had been dislodged.
Footage of the event was shared by Swiss journalists on social media.
12 farm buildings, including barns and stables, had been destroyed, according to police statement.
The Austrian foreign ministry confirmed that a couple from the country was missing, while Blick reported that four of those being searched for were Germans.
Two hundred villagers were evacuated on Wednesday.
"People are immediately gone, leaving everything behind them".
"The noise was deafening", one of them said.
Residents were being housed in nearby hotels, including in the village of Castasegna on the Italian border.
There are still eight people whose whereabouts remain unknown following the landslide on Wednesday on the Piz Cengalo mountain at the Italian border.
Police said the landslide struck a remote valley in the south-east of the country yesterday.
The alarm system in Bondo was installed after a huge landslide on the slopes of Piz Cengalo in 2012 which crashed into an uninhabited valley.