More than 12 hours after the INS Sindhurakshak caught fire and sank in a Mumbai dock, rescue dive crews have been unable to make contact with the 15 sailors and three officers, or enter the vessel.

Defence Minister AK Antony confirmed some sailors had been killed, but did not say how many.

“I’m saddened by the incident as many Navy personnel lost their lives,” he said.

Shortly after midnight on Wednesday morning a massive explosion started a fire onboard the diesel-powered Sindhurakshak as it sat in a high-security naval dock.

Sailors reportedly jumped off the vessel as it began to sink, but those trapped below decks were unable to escape.

“There are some people who are trapped on board, we are in the process of trying to rescue them, we suspect it to be in the range of 18,” Navy spokesman PVS Satish told news agencies.

“We will not give up until we get to them.”

As dive crew boats and fire tenders surround the stricken Sindhurakshak, only a tiny portion of the vessel is visible above the water line.

Injured sailors who escaped the submarine were taken to the Ashvini naval hospital in Colaba.

Indian media has reported that the Sindhurakshak was fully-laden and armed with warheads at the time of the explosion, but it appears a fault with the submarine’s battery system, which has caused a fire in the vessel before, could be the cause of the explosion.

Naval sources have said the batteries were being charged at the time of the explosion, and that leaking hydrogen may have led to a dangerous build-up of the highly flammable gas.

In February 2010, a sailor on board the Sindhurakshak was killed by a fire in the submarine’s battery compartment. The submarine was sent to Russia for an $80 million overhaul and upgrade, from which it had returned only a few months ago.

A second submarine, the Sindhuratna, was also damaged in Wednesday’s catastrophe.

The initial explosion, captured by witnesses on phones, sent a massive fireball into the sky above Mumbai, and smoke from the resultant fire could be seen across India’s largest city for several hours.

The Sindhurakshak is one of 10 Kilo-class submarines India bought from Russia between 1986 and 2000. They are equipped with Russian Club-S cruise missile systems, but they do not have escape capsules, as some Soviet-era submarines do, which might have helped the trapped sailors escape.

Sindhurakshak is a Sanskrit word meaning ‘defender of the Indus’, a major subcontinental river.

Before the explosion, the past week had been a proud one for the Indian Navy.

On Monday, it unveiled the country’s first indigenous aircraft carrier, the 37,500-tonne, 260-metre INS Vikrant.

And last Friday, the atomic reactor on board India’s first nuclear-powered submarine, the INS Arihant, was activated.

Prime Minister Manmohan Singh described the development as “a giant stride in the progress of our indigenous technological capabilities”.

The explosion on board the Sindhurakshak came just a day before India’s largest national celebration, Independence Day, marking 66 years since its emancipation from Britain.

With agencies