The blast ripped through the tunnel on Monday at Samardung village in the Himalayan state of Sikkim, where workers were carrying out construction work on the 500-megawatt Teesta hydroelectric project, Anupama Tamling, a civil official in the area, told reporters.
Survivors who managed to escape told authorities they heard a loud explosion before debris came crashing through the tunnel, trapping dozens inside, she said.
As news of the accident spread, six officials from the state-owned National Hydroelectric Power Corporation, which is building the project, entered the tunnel to assist in rescue efforts but became trapped themselves, officials said.
"We have recovered 10 bodies so far and 17 others are feared trapped," local police chief Sonam Dolma told reporters.
Rescue teams, including the National Disaster Response Force (NDRF), have been struggling to reach those trapped because of hazardous conditions inside the tunnel, where toxic gases have accumulated.
"We found toxic gases such as methane, carbon monoxide and hydrogen sulphide inside the tunnel," NDRF official Nitin Kumar told reporters at the scene.
He said rescuers were using specialised breathing equipment as they tried to search for survivors amid dangerous air conditions.
Construction accidents are not uncommon in India, where rapid infrastructure growth often collides with fragile geology, construction lapses, extreme weather and safety standards.
In the Himalayan region, fragile mountains, seismic activity and unpredictable underground conditions make tunnel projects especially risky.
