The operation to rescue 41 trapped labourers from Uttarakhand's Silkyara tunnel was halted on Thursday night after auger machine broke down, media reports said.

A top National Disaster Management Authority official said the operation should not be given a timeline.

"Many experts are giving opinions that they might be rescued today evening, tomorrow morning but remember these operations are like a war. These operations should not be given a timeline. In wars, we don't know how the enemy will react. Here Himalayan geology is our enemy. From which angles the tunnel has collapsed we don't know," Lt. General (Rtd) Syed Ata Hasnain, a member of the National Disaster Management Authority (NDMA), was quoted as saying by Hindustan Times.

He termed the operation as challenging.

Meanwhile, the drilling operation is expected to begin shortly on Friday.

The labourers have been trapped in a 400-metre buffer zone of the unfinished main tunnel since November 12 when a portion of the 4.5-km tunnel on the Brahmakal-Yamunotri National Highway collapsed.

The tunnel - meant to join Silkyara and Dandalgaon in Uttarkashi - is a part of the Char Dham project.

On Monday morning, a wider pipe was installed to deliver a wider variety of food and medicine to the trapped people. Earlier, only dry fruits could be sent in.

The rescue ops involved an international tunneling expert team and a robotics team from defence research organization (DRDO).

Over the last week, several plans to cut through the 40-meter rock wall fell through.