In the Kursk sinking and subsequent accidents in Russia’s submarine fleet, the navy has been slow in acknowledging the gravity of emergencies, the scale of human loss or the environmental threat.

The military announced the latest fire and casualties on Tuesday, but said the accident happened a day earlier. It said the sailors had died from smoke inhalation. The statement offered no explanation for the delayed announcement.

It was not clear if the vessel was submerged at the time of the fire, and the military did not specify its location, other than to say it had been within Russian territorial waters. The Severomorsk base is on the Murmansk Fjord, which opens to the Barents Sea.

The Ministry of Defense issued a statement describing the stricken vessel as a “scientific experimental deep water apparatus intended to study the natural environment and sea floor.”

But some Russian media indicated that the vessel was a spy submarine. In a possible indication of the importance of the vessel or its mission, Mr. Putin said seven of those who died were captains and that two of the dead had received high military honors.