Russian President Vladimir Putin disclosed on Thursday for the first time that the secret military submarine hit by a fatal fire three days ago was nuclear-powered, prompting the Defence Minister to assure him its reactor had been safely contained.

Mr. Putin revealed that the submarine had been nuclear-powered by asking Defence Minister Sergei Shoigu during a Kremlin meeting about the reactor’s condition after the fire. “The nuclear reactor on the vessel is completely isolated,” Mr. Shoigu told Mr. Putin, according to a Kremlin transcript. “All the necessary measures were taken by the crew to protect the reactor.”

Fourteen crew were killed in the accident.