The fate of 18 persons onboard the submarine is being ascertained, a Navy official 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, a defence spokesperson said.
Fire tenders from the Naval dock yard as well as the Mumbai Fire Brigade were immediately pressed into action, he said.
However, due to the explosion, the submarine has submerged at the dock with only a portion visible above the surface.
A Board of Inquiry is being instituted to investigate into the causes of the accident, he said.
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