A landslide blocked the Samardung tunnel entrance at Jholungey in Sikkim's Namchi district on Monday afternoon, killing seven people.
Approximately 27 workers, including 21 from Patel Engineering and six from NHPC, are feared trapped inside the under-construction tunnel.
Toxic methane gas emanating from disrupted subterranean rock layers has severely hindered rescue operations, causing some personnel to lose consciousness.