An avalanche on one of Russia’s highest mountains has killed 11 climbers and left six others missing, rescue services said Monday.

Rescuers received reports of the avalanche on Mount Mizhirgi, around 5,000 metres (16,400 ft) tall, at around 5 pm (1400 GMT) on Sunday.

Unverified video on social media showed what appeared to be a mound of snow and ice crashing down a mountainside in the area, as people in the background could be heard swearing.

“Tragically, 11 people were killed. The fate of six climbers remains unknown,” the local branch of Russia’s emergency ministry said in a statement.

“Search and rescue operations are being complicated by difficult terrain and the risk of avalanches,” it added.

A group of 33 climbers were in the area at the time.

Ten managed to evacuate on their own, while six had to be transported out by rescue services, according to the local branch of Russia’s emergency ministry.

Prosecutors have opened a case into criminal negligence.

Mount Mizhirgi, near the Georgian border, is Russia’s eighth-tallest mountain.

Surrounded by pristine forest and glaciers, the area is popular with hikers.