Four men, including three standing trial for the assassination of former Punjab chief minister Beant Singh, breached a multi-tier security cordon and escaped from the high security Burail jail in Chandigarh, police sources said on Thursday.

Jagtar Singh Hawara, Jagtar Singh Tara and Paramjit Singh, along with their helper Ved Singh, escaped from their barrack by digging a nearly 100 feet long tunnel to a spot outside the prison. They slipped out some time after the routine check on Wednesday night.

The three are among nine accused in the assassination of Beant Singh, who was killed by a human bomb outside the Punjab and Haryana secretariat on August 31, 1995. Seventeen others also perished when Dilawar Singh, a police constable, blew himself up in a car identical to the vehicles in the chief minister's motorcade.

The police have alerted neighbouring states and is keeping a close watch on railway stations, airports and places known to the escapees.

Describing the incident as 'unfortunate', Punjab Chief Minister Amarinder Singh expressed hope that the four would soon be nabbed.

Counsel for the accused Amarjit Singh Chahal, however, alleged that the authorities had 'cooked up' the entire incident saying digging a tunnel inside the high security prison was 'unbelievable'.