A blast followed by a major fire in the Indian submarine INS Sindhurakshak, which was docked in Mumbai, has trapped 18 sailors.
The explosion took place at about midnight, but the cause of the blast is yet to be ascertained.
Rescue teams have rushed to the high-security naval dockyard. Some of the navy personnel on the gutted submarine could have been killed, suggest local reports. However, there is no official confirmation.
Oxygen supply is limited inside the Kilo-class vessel and it has been nearly ten hours since the fire broke out.
The diesel-powered submarine was fully operational. Torpedoes and missile warheads are believed be onboard the Russia-made submarine.
No other ship or submarine has been damaged in the incident. Visuals of the fire suggest multiple explosions could have taken place at the 16-year old vessel.
"Due to as-yet-unknown damage suffered as a result of the explosion, the submarine has submerged at her berth with only a portion visible above the surface. 18 persons were on board the submarine at the time of the accident and efforts are on to ascertain the safety of the personnel and salvage of the submarine," the Indian navy said in a statement.
The incident is likely to be a setback for India over its defence capabilities. The vessel had recently returned to India following overhaul in Russia. New Delhi is thought to have spent nearly $80m (£51m) for the refurbishment with improved warfare mechanisms.
"I heard a loud sound and then I saw smoke and fire. I was at work when I heard a loud sound. The sky had turned red," an onlooker told the CNN-IBN news network.