At least 12 workers were killed after a portion of an under-construction tunnel collapsed following a gas explosion in India’s north-eastern Sikkim state, officials said.
Rescue operations are continuing to locate 13 others who are reported to be trapped in the tunnel since Monday’s collapse.
The National Disaster Response Force (NDRF) said the incident happened about 1.5km (0.9 miles) inside the tunnel.
The under-construction tunnel, designed to carry water from the nearby Teesta River to a hydropower plant’s turbines, is part of a larger project in the Himalayan state being developed by the state-owned National Hydroelectric Power Corporation (NHPC).
The accident happened “after [a] sudden burst of suspected methane gas trapped/embedded inside the rocks which led to [an] explosion generating dense fumes and toxic gases”, NHPC said in a statement.
In a separate statement, the Sikkim government said the accident was caused by “methane gas combustion”.
Methane is a naturally occurring gas that can sometimes be trapped underground. If tunnelling releases it into a confined space and it comes into contact with a spark or another ignition source, it can ignite or explode, a government official said.
There were 27 people working in different sections inside the tunnel – of which two managed to escape after the explosion and 25 were trapped.