Rescue teams have been struggling to reach the 17 people trapped because of hazardous conditions inside the tunnel.

At least 10 people have died and 17 others remain missing after a suspected gas explosion triggered the collapse of a tunnel under construction at a hydropower project in India’s northeastern state of Sikkim, officials said.

The blast ripped through the tunnel on Monday at Samardung village, near the border with Bhutan and Tibet in the Himalayas, officials said.

Multiple workers were carrying out construction projects on the 500-megawatt project for the state-owned National Hydroelectric Power Corporation (NHPC) on the Teesta River. Survivors told authorities they heard a loud explosion before debris came crashing through the tunnel, trapping dozens inside.

NHPC said in a ⁠statement that a sudden burst of gas suspected to be methane trapped or embedded in rocks in the tunnel caused the blast.

As news of the accident spread, six officials from the NHPC 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 said.

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, especially in the Himalayan region, where seismic activity and unpredictable underground conditions make tunnel projects especially risky.

In 2023, 41 workers were trapped for 17 days before being rescued after a section of a tunnel in northern Uttarakhand state collapsed.

Experts told AP that the Teesta River basin, where the hydropower project is being built, lies in a seismically active region marked by young, fragile rock formations riddled with fractures and underground pockets that can trap and accumulate gases, triggering explosions.