The incident happened off the Mumbai coast on the kilo-class submarine while it was on a routine training exercise.

The two naval officers who were feared dead in a fire on INS Sindhuratna, 50 nautical miles off Mumbai on Wednesday morning, might have been in one of the compartments of the Russian-origin Kilo-class submarine, Navy sources told The Hindu.

The sources said a thick layer of smoke was reported in compartment three of the sailors’ accommodation section around 6 a.m. The fire was brought under control by the crew, and the compartments were sealed. Ninety-four crew members were on board when the incident occurred.

All seven sailors, who were trapped inside the compartment, complained of asphyxiation. They were airlifted by a Navy helicopter and admitted to INS Ashvini, a naval hospital in south Mumbai. A Lieutenant and a Lieutenant Commander were listed as missing.

The Commodore Commanding Submarine (West), S.R. Kapur, was rescued along with the crew.

While the cause is yet to be ascertained, the sources say a preliminary assessment revealed that an explosion in the battery compartment could be the reason. An inquiry has been ordered by the Defence Ministry.

“Two personnel have not yet been located, and all efforts are being made to locate them. All other crew members are on board and safe,” a Navy release said.

The submarine was at sea on a training and inspection exercise. It was being tested for seaworthiness after a refit in December last. It had to undergo two tasks — one at the harbour and the other at sea. The harbour exercise had been completed, and the fire broke out when the sea exercise was under way. The submarine was carrying no ammunition, the sources said.

“Naval ships were despatched by the Western Navy Command Office and are in the area to provide assistance to the submarine. The submarine is safe, and it does not have any weapons,” the release said. The submarine was being towed to the Mumbai harbour.