Confirming for the first time that the accident happened on a nuclear-powered vessel, Shoigu told President Vladimir Putin that the crew took measures to protect the reactor from the flames, and that the submersible can be repaired.

"We identified the main reason for the accident, a fire in the battery compartment, which later spread," Shoigu said in his report to the president after his trip to Arctic port Severomorsk, where the unnamed vessel is currently docked.

Kommersant daily on Wednesday said, citing sources close to the crew, that the investigation is considering a powerful short circuit as a reason for the fire.

"The nuclear installation on this apparatus is completely isolated and unmanned on this vessel," Shoigu said. "In addition, the crew carried out all necessary measures to protect the installation, and it is in working order."

Shoigu earlier said that members of the crew sealed the hatch after evacuating a civilian from the compartment on fire, isolating the rest of the ship.

The name of the vessel has not been officially published, and the Kremlin said that much of the information surrounding the accident would be kept classified.

Russian media, citing sources, have named the vessel as Losharik, the unofficial name for a project run by the top-secret Main Directorate of Deep Sea Research (GUGI).

Russian newspaper Fontanka on Tuesday reported 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.

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

However Kommersant said that as the AS-31 remains 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.