The Indian Navy's Sindughosh-class submarine, INS Sindhurakshak (S63), has submerged off the naval dockyard in Mumbai, following an explosion shortly after midnight on 14 August 2013.
The submarine, which recently completed a $80m (£52m) modernisation programme in Russia, was reportedly carrying a crew of 18 onboard at the time of incident.
Even though an investigation is under way to establish the cause, unidentified sources attribute the explosion to a defective battery, which resulted in similar submarine accident in Visakhapatnam in 2010.
The fully-armed submarine also reportedly damaged, another Sindhughosh-class submarine, INS Sindhuratna (S59) was stationed at the same dockyard by setting off two onboard torpedoes.
Meanwhile, the crew, whom the navy earlier insisted to be trapped, are now feared to be dead.
Confirming the death of sailors, Indian Defence Minister AK Antony told reporters that "I feel sad ... about those navy personnel who lost their lives in the service of the country."
"It's a great loss to us... it's the greatest tragedy of recent times," Antony added without giving further details of the accident.
Indian navy spokesman, PVS Satish, said the submarine's front portion has been severely damaged in the explosion.
Powered by diesel-electric propulsion with two diesel generators and one propulsion motor, the type 877EKM submarine features MGK-400 sonar, active radar with a target separation system as well as a radar warning receiver and a direction finder.
The Sindughosh-class submarines have been designed to support anti-submarine (ASW) and anti-surface ship (ASuW) warfare missions.
Image: INS Sindhurakshak (S 63) stationed off the port city of Mumbai, India. Photo: PH1 (NAO) Chris Desmond, USN.