At least nine people have died and 15 others remained trapped after a tunnel collapsed at an under-construction hydroelectric project in India’s remote northeast, officials said on Tuesday.

The incident occurred on Monday in the Himalayan state of Sikkim, where workers were constructing a tunnel for a state-run hydropower project on the Teesta river.

“We are estimating around 25-27 people were trapped,” Rajiv Roka, director of the Sikkim State Disaster Management Authority, told AFP.

“Nine bodies have been recovered.”

The collapse was likely triggered by a blast, the cause of which was yet to be ascertained.

The region has seen torrential monsoon rains in recent days.

Roka said authorities were “not sure what caused the blast because it was deep in the tunnel”, adding that “it was most probably natural”.

Toxic methane gas leaking from underground rock formations was hampering rescue efforts, officials said, allowing only teams carrying oxygen equipment to enter.

“Aggressive rescue operations are under way, but the problem is there’s a lot of gas present there,” Roka said.

A specialised team trained in mining accidents had been flown in to aid the rescuers, he added.

Accidents on large infrastructure construction sites are common in India, but ecologists say excessive development projects have led to more disasters in the fragile Himalayan areas.

In 2023, 41 workers were rescued after 17 days being trapped under a Himalayan road tunnel that had collapsed in northern Uttarakhand state.