Forty workers have been trapped in the debris for nearly 150 hours following the collapse of a tunnel in Uttarakhand's Uttarkashi last Sunday, media reports said.

The rescue operation was halted last evening after a "cracking sound" was heard and the drilling machine developed a snag.

The 40 workers have been trapped inside the 4.5-km Silkyara tunnel since Sunday morning when a part of the under-construction structure near Uttarkashi collapsed following a landslide.

"During the execution of the work at around 2:45 pm, a large scale cracking sound was heard by officials and the team working inside the tunnel and created a panic situation in the tunnel as well as to the team working," the National Highways & Infrastructure Development Corporation Limited (NHIDCL) said in a statement.

It said there's a "strong possibility that a further collapse may take place and accordingly pipe pushing activity has been stopped".

The rescuers need to drill up to 60 metres to insert 800 mm and 900 mm diameter pipes with the help of a giant drill machine to create an escape passage for the stranded workers.

"Positioning of the fifth pipe was under progress. It was reported that the machine was not able to push further as it was getting lifted and the bearings of the machine were damaged. 

"It was planned to fix the machine by anchors to avoid uplifting. It was decided and agreed by the special agency executing the work that further pushing of the pipe will be feasible only if the machine is placed with the same inclination as the pipe," the statement said.

Elite rescue teams from Thailand and Norway, including the one that successfully rescued the trapped children from a cave in Thailand in 2018, have joined the rescuers to aid in the ongoing rescue operation.

The round-the-clock rescue work is being helmed by 165 personnel from multiple agencies, including NDRF, SDRF, BRO, and the ITBP.