Rescuers raced to free around 35 Indian construction workers trapped in a tunnel, two days after the hydroelectric dam they were helping to build was swept away by a wall of water from a collapsed glacier that barrelled down a Himalayan river.

According to the Sky News, so far, 31 bodies have been recovered but officials fear for the 165 who are missing at the two hydropower plants at Rishiganga and Dhauilganga.

They have only managed to reach about 100 meters into the mile-long tunnel at Tapovan until now.

The eight-metre-high tunnel is packed with debris and slush, and earthmovers have been continually removing it since operations began on Sunday.

Packing rocks, dirt and construction debris and thought to have been triggered when a glacier lake fed by India's second highest peak, Nanda Devi, collapsed, the flood swept down the Dhauliganga river on Sunday.

Officials said most of those still missing were shift workers at either the Tapovan Vishnugad hydroelectric project, where the tunnel was situated, or at Rishiganga, a smaller dam which was swept away in the flood.

Over 1,000 military, paramilitary and specialists have been operating day and night at the site. Sniffer dogs are waiting to be taken in.

Manoj Rawat of the Indo-Tibetan Border Police, which is spearheading the operation, told Sky News: "All efforts are being made to rescue people trapped inside. We are working towards that."

(With input from agencies)