The driver appeared to have lost control of the vehicle, swerving off the road and sending the bus plummeting 25m into a ravine, according to officials.

The incident took place at around 3.20am, the local interior ministry said.

It published images showing the bus overturned next to a pool of muddy water, its wheels facing upward, while another showed a twisted metal barrier on the road above.

“The road accident occurred on the industrial road of the Denisovsky Mining and Processing Plant,” it added.

The plant mines and processes coal.

Investigators said they had opened a probe into criminal negligence.

Industrial accidents are common in Russia, with investigators often pointing to lax safety standards.

Local authorities declared a day of mourning for Tuesday.