INDIAN divers and engineers are struggling to refloat a stricken submarine in a Mumbai military shipyard after it exploded and sank with 18 crewmen on board.

The fully-armed INS Sindhurakshak, returned by its original manufacturer Russia earlier this year after a major refit, went up in flames in a dock yesterday and sank to the bottom of the harbour.

Defence ministry spokesman Sitanshu Kar said: "We have not got the bodies so we can't say they are dead. But we are fearing the worst."

Navy chief D.K. Joshi said no sign of life had been detected on board.

"While we hope for the best, we have to prepare for the worst," he told reporters in Mumbai, adding that there was a possibility some crew might have found air pockets but "the indicators are negative".

Divers had opened the main hatch and are expected to secure two water-tight compartments and begin pumping out the water to bring the vessel to the surface.

The disaster is considered the Indian navy's worst since the sinking of a frigate by a Pakistani submarine in 1971 and premier Manmohan Singh is expected to speak about it in his annual Independence Day speech on Thursday.

Defence Minister A.K. Antony described the explosion as the "greatest tragedy in recent time".

The blast came days after New Delhi trumpeted the launch of its first domestically-produced aircraft carrier and the start of sea trials for its first Indian-made nuclear submarine, described by Singh as a "giant stride" for the country.

The world's biggest democracy has been expanding its armed forces rapidly to upgrade its mostly Soviet-era weaponry and respond to what many in India perceive as a growing threat from regional rival China.

Amateur video footage showed a fireball in the forward section of the Sindhurakshak, where torpedoes and missiles are stored as well as the battery units.

A board of inquiry has been established to probe all possible explanations for the explosions including sabotage, but "the indicators at this point of time do not support that theory", Joshi said.

Other sailors on vessels berthed near the INS Sindhurakshak were admitted to a navy hospital in Mumbai with burns.

The disaster had echoes of a tragedy in Russia in 2000 when the Kursk nuclear submarine sank in the Barents Sea with the loss of all 118 crew on board.

Russia is still the biggest military supplier to India, but relations have been strained recently by major delays and cost overruns with a refurbished aircraft carrier, the INS Vikramaditya.

Major #explosion at #MumbaiNavalDockyard.. call 9821535935 for more details and live videos... pic.twitter.com/U6bTjHzBgZ — VIKALP SHAH (@shahvikalp) August 13, 2013

Originally published as Rush to raise submarine in survivor hunt