Mumbai:
The INS Sindhurakshak, a diesel-powered submarine, exploded and sank early Wednesday in a dock in Mumbai, with the 18 sailors on board trapped inside the vessel.
Sources say the submarine was fully armed when the explosion took place in the weapons' bay. The fire resulted in the detonation of torpedos or missiles on board.
The 16-year-old submarine had been returned by Russia less than a year ago after a major refit and was submerged after the accident, the navy said.
Grainy and shaky amateur video footage taken by a witness showed the fierce explosion which lit up the sky at the naval dockyard shortly after midnight.
Sources said divers had been deployed to locate the trapped sailors, but at 11 am, contact had yet to be established with those missing and the divers had not been able to enter the boat.
"Eighteen sailors were on board the submarine, they have not been evacuated yet," navy spokesman PVS Satish told news agency AFP.
"There were two to three explosions and the night sky lit up briefly," said eyewitness Dharmendra Jaiswal, who manages a public toilet near the dockyard and was sleeping there overnight.
In February 2010, the INS Sindhurakshak also suffered a fire while docked in Visakhapatnam city in southern India, killing a 24-year-old sailor and leaving two others with burns.
INS Sindhurakshak is a kilo-class submarine which normally operates with a crew of 53 people and can sail on its own for 45 days, the Indian navy website says.