Eighteen Indian sailors are trapped and a number have been killed after an explosion and fire on a diesel-powered submarine berthed at a base in Mumbai.
Defence Minister AK Antony said on Wednesday that crew members inside the Russian-built INS Sindhurakshak have died, but gave no details of numbers.
Mr Antony said the explosion on Tuesday was likely to have been an accident, but an investigation is underway, the BBC reports.
Indian navy officials said the submarine has sunk, with only a portion visible above the surface.
The Russian-built submarine was upgraded recently at a cost of $US80 million by Russian firm Zvyozdochka. It said the vessel had been fully operational when it was returned to India in January this year.
The INS Sindhurakshak is one of the 10 Kilo-class submarines bought between 1986 and 2000 and carried Russian Club-S cruise missile systems.
The submarine was sent to Russia for the refit in 2010 after a sailor on board was killed by a fire that broke out in the battery compartment while it was docked at the Vishakhapatnam naval base in February that year.