GANGTOK: At least nine workers have died and around 15 remain trapped following a tunnel collapse at an under-construction state-run hydropower project on the Teesta River in Sikkim, India.

The incident, triggered by a deep underground blast likely linked to heavy monsoon rains, occurred on Monday while 25 to 27 workers were inside.

Specialized mining disaster teams have been flown in to lead aggressive rescue operations, but efforts are severely hindered by high concentrations of toxic methane gas leaking from rock formations, requiring responders to rely on specialized oxygen gear.