The Birch glacier in Switzerland's southern Valais region collapsed on Wednesday, sending a mass of rock, ice and scree hurtling into the village of Blatten in the valley below.

The huge pile of glacier debris, stretching across two kilometres (1.3 miles), has blocked the river Lonza and created an artificial lake, potentially posing threats of flooding in the Lotschental valley below.

"We absolutely cannot let our guard down," Raphael Mayoraz, a geologist working with the Valais canton, told public television.

Mayoraz spoke of a "favourable scenario" but warned that "we are not certain that the danger is averted".

Authorities said that they had proceeded with the evacuation of 16 people from two villages located downstream from the disaster area and that they would remain alert in the coming weeks.

Authorities are studying evacuation plans and have warned residents who could be affected.

The collapsed glacier largely destroyed most of Blatten, which had been home to 300 people and was evacuated last week due to the impending danger.

One 64-year-old man, believed to have been in the danger zone at the time, remains missing. A police spokesman said difficult conditions had forced the search to be called off on Thursday.

An artificial dam in the village of Ferden, just below, has been emptied and should be able to contain any downward rush of water if it happens, authorities said Thursday.

What remains of the village of Blatten is being submerged beneath the accumulating water of the Lonza river.

Where the Birch glacier used to sit, there is now a gaping hole in the mountainside.