The trapped personnel have not yet been sighted or recovered," a Navy release said here.
"The diving efforts are hampered by poor visibility inside submarine which is filled with water, extremely restricted access and displacement of most equipment from their original location," the release said.
The heat of the explosion has melted parts of the internal hull deforming the submarine hatches and has prevented access to the compartments.
"Heavy duty pumps are being used to pump out the water from the submarine. There has been large scale ingress of sea water into the submarine due to the explosion," it said, adding diving and salvage operations are continuing.
In one of the worst disasters to have struck the Indian Navy, a series of explosions rocked the Russian-made Kilo class submarine at the dockyard here early Wednesday, sinking it partially in the shallow sea, in which 18 personnel including three officers are feared killed.
Yesterday, Navy Chief Joshi Admiral D K Joshi, who accompanied Defence Minister A K Antony, did not rule out the possibility of a sabotage but said that the indicators so far do not support such a theory. He had also indicated that there was little hope of survival of 18 personnel on board the submarine, which exploded and sank at the naval dockyard in Colaba.
The diesel-electric submarine was commissioned into the Indian Navy in 1997 at a cost of around Rs 400 crore and had gone through a Rs 450-crore extensive upgrade in Russia.
The 2300-tonne submarine, powered by a combination of diesel generators and electric batteries, had potent weapons package including the anti-ship 'Club' missiles.