In a major setback to the Navy´s ambitious modernization drive, the fully armed INS Sindhurakshak, returned by Russia earlier this year after a major refit at an estimated cost of $80 million exploded in a Mumbai dock.
A Navy spokesperson said divers had managed to pry open the main hatch of the Russian-built submarine, more than 12 hours after the incident on Wednesday, and were trying to find their way through the vessel.
India's defence minister A.K. Antony described the explosion as the "greatest tragedy in recent time", as media reports said the disaster was the worst one in over three decades.
"I feel sad about those navy personnel who have lost their lives in service of the country," Antony said in New Delhi, adding that it was unclear how many had perished.
Indian Navy chief Admiral D K Joshi said he is hopeful about the sailors while preparing for the worst.
The submarine was fully loaded with missiles and torpedoes when the explosion led to a massive fire on board INS Sindhurakshak at the Naval dockyard in Mumbai.
The submarine sank, with at least 10 people on board soon after the explosion.
Safety mechanisms to prevent disasters did not work, the naval chief admitted at a press conference in Mumbai, while adding that an inquiry on the disaster is to be completed within four weeks.
"Quite obviously the safety mechanisms have not functioned," the Navy Chief said.
The Navy chief said that the cause of the catastrophe was not yet known.
However, he has ruled out the possibility of sabotage.
Meanwhile, naval divers managed to cut through thick layers of metal to enter the INS Sindhurakshak at 4 pm.
The 16-year-old submarine was reportedly carrying heavyweight torpedoes and "Klub" anti-shipping and land attack missiles.
The incident reportedly occurred when INS Sindhurakshak was 15 to 20 metres away from the jetty in the sea.
Earlier the Indian Navy in a statement said,"An explosion resulting in a major fire took place on board INS Sindhurakshak, a Kilo class submarine of the Indian Navy, shortly after midnight on 14 August 2013."
"Fire tenders from the Naval Dockyard as well as the Mumbai Fire Brigade were immediately pressed into action."
"However, 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," the statement said.
The Indian Navy said: "About 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."
"A Board of Inquiry is being instituted to investigate into the causes of the accident," it said.
In February 2010, a fire broke out aboard INS Sindhurakshak while she was in Visakhapatnam. One sailor was killed and two others were injured.
Navy officials reported that the fire had been caused by an explosion in the submarine´s battery compartment.
INS Sindhurakshak was built in 1997 at Saint Petersburg, for anti-submarine and anti-ship warfare, defending naval bases, securing coasts and sea lanes of communication, reconnaissance and maritime patrol operations.
The vessel had been sent to Russia after the Visakhapatnam accident for a mid-life upgradation in 2010.
She was handed over to India earlier this year after nearly two years of overhaul and refitting in Russia at the cost of 80 million dollars (Rs. 480 crore). The submarine was expected to serve the navy for at least 10 more years.
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