(CNN) A deepwater submersible that caught fire Monday was nuclear-powered, according to Russia's President Vladimir Putin.

Putin's comments came during a conversation with the country's defense chief, who confirmed that the incident was caused by a fire in a battery compartment. The Kremlin had initially been reluctant to release information about the blaze, which left 14 sailors dead due to smoke inhalation, calling it a state secret.

The submersible, which is smaller than a submarine and has limited power reserves, was carrying out research in Russian territorial waters at the time of the fire and is now at the naval base in Severomorsk on the Barents Sea.

According to a Kremlin transcript released Thursday, Putin asked Defense Minister Sergei Shoigu about the status of the vessel's "nuclear power unit."

"Mr. President, the nuclear power unit has been sealed off," Shoigu, who had visited Severomorsk, replied.

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