Kronshtadt, Russia -- Russian President Vladimir Putin on Thursday confirmed reports that the navy's deep-sea submersible vessel that caught fire on Monday, killing 14 seamen, was nuclear-powered. Russian officials previously declined to release any details of the vessel, citing its secret mission.

Putin met Thursday with Defense Minister Sergei Shoigu, who had returned from the Navy's Arctic base of Severomorsk, and asked him about the submersible's nuclear reactor. Shoigu said the reactor is "completely isolated now" and it is "in full working order."

The blaze erupted at the vessel's battery compartment and spread, Shoigu said in televised remarks. The Defense Ministry earlier said 14 people have been killed and that there was an undisclosed number of survivors.

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Details remained scare about the fire and the ship, which was on a mission to measure sea depths in Russia's territorial waters in the Barents Sea.

The ministry didn't name the vessel, but Russian media reported it was the country's most secret submersible, a nuclear-powered research submarine called the Losharik intended for sensitive missions at great depths.

Navy sailors attend a religion service to commemorate the crew members that were killed on one of the Russian navy's deep-sea research submersibles at Kronshtadt Navy Cathedral outside St.Petersburg, Russia, Thursday, July 4, 2019. Dmitri Lovetsky / AP

Hundreds of sailors gathered on Thursday at Russia's main naval cathedral on an island in the Gulf of Finland just off St. Petersburg to mourn the deaths of the sailors. The seamen and the priests who formerly served in the navy lit candles and took part in the prayer at the Kronshtadt Navy Cathedral.

The Russian newspaper Fontanka reported Tuesday that a military base in Saint Petersburg published an announcement about the death of the crew of the AS-31, a nuclear-powered deep-water sub that is part of the Losharik project.

Previously, media identified the sub as AS-12, also part of Losharik.

Map shows location of Russian Arctic port of Severomorsk Google Maps

Shoigu said he hopes that the submersible "can be restored in a relatively short time."

However, Kommersant said that as the sub remains part of a top-secret project, even military investigators have no access to the dock where the vessel is located and crew members cannot be formally identified.