On Wednesday the Birch glacier in Switzerland's southern Valais (Wallis) region collapsed, sending tons of rock, ice and scree hurtling down the mountain slope and into the valley below.

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

One person, a man aged 64, believed to be in the affected zone at the time, remained reported missing. A police spokesman said the difficult conditions had forced the search for him to be called off Thursday afternoon.

Authorities declared a local state of emergency Thursday as they monitor the situation after the huge pile of glacier debris, stretching some two kilometres (1.25 miles), blocked the river Lonza.

"There is a serious risk of an ice jam that could flood the valley below," Antoine Jacquod, a military security official, told the Keystone-ATS news agency.

"We're going to try to assess its dimensions today," added Jacquod.

Authorities said an assessment would be made at 6:00 pm (1600 GMT) from the nearby village of Ferden as to whether an approach to the zone could be attempted.

As a precaution, 16 people were evacuated late Wednesday from the villages of Wiler and Kippel located downstream from the disaster area in the Loetschental valley, known for scenic views stretching some 30 kilometres and home to around 1,500 people living in traditional villages.

- 'Not very stable' -

"It's like a mountain, and of course, it creates a small lake that gets bigger and bigger," explained Raphael Mayoraz, the cantonal official in charge of natural hazard management, Wednesday evening.

An artificial dam was preemptively emptied to receive the water pushed back by the wall of ice, earth and rubble.

Were that water to overflow from the dam, authorities would need to consider evacuating the valley.

The level is rising 80 centimetres (31 inches) an hour, according to Swiss daily Le Temps.

The huge amount of debris makes the peaks overlooking the valley appear smaller, while a gaping hole can be seen on the side of the mountain from where the land slid down into the valley.

The Valais cantonal government has meanwhile asked the army -- which set up a base camp in Turtmann Wednesday and is readying helicopters for making an approach to the affected zone -- to provide clearing equipment and pumps to secure the riverbed.

"The deposit ... is not very stable, and debris flow is possible within the deposit itself (which) makes any intervention in the disaster area impossible for the time being," cantonal authorities stated, adding there was risk on both sides of the valley.

They also warned of a risk of torrential lava within the debris deposit itself, preventing for the time being any intervention in the disaster area.

- Seismic event -

YouTube footage of the collapse showed a huge cloud of ice and rubble hurtling down the mountainside, into the valley, and partially up the mountain slope on the other side.

The force was such that Swiss monitoring stations registered the phenomenon as a seismic event.

According to Mayoraz, "three million cubic metres of rock fell suddenly onto the glacier, carrying it with them" down into the valley.

Warming temperatures have both shrunk the Alps' glaciers and have made them more unstable.

Swiss glaciers, severely impacted by climate change, melted as much in 2022 and 2023 as between 1960 and 1990, losing in total about 10 percent of their volume.

In late August 2017, approximately 3.1 million cubic meters of rock fell from Pizzo Cengalo, a mountain in the Alps in Graubuenden canton, near the Italian border, claiming the lives of eight hikers.

Some 500,000 cubic metres of rock and mud flowed as far as the town of Bondo, causing significant material damage but no casualties.

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