(Photo: VCG)

MOSCOW, July 3 (Xinhua) -- Some details about Monday's fire on a Russian deep-water apparatus, which killed 14 people on board, are classified and will not be made public, the Kremlin said Wednesday.

"There is information that belongs to the state secret category. Therefore, this is an absolutely normal practice when such information is not disclosed in any way," Kremlin spokesman Dmitry Peskov told reporters.

On Tuesday, the Russian Defense Ministry said in a statement that 14 submariners were killed by combustion gas in a fire on a deep-water research apparatus in Russian territorial waters.

Later in the day, Russian President Vladimir Putin summoned Defense Minister Sergei Shoigu and ordered him to go to the Severomorsk base of Russia's Northern Fleet in person to ensure a thorough investigation.

The submariners on board were unique military specialists and highly skilled professionals who conducted an important survey of the Barents Sea floor, Shoigu said Wednesday at a meeting with members of an investigative commission.

Seven of the victims were first rank captains, corresponding to the rank of a colonel in the army, of which two held the country's highest military award "the Hero of Russia," while another three were second rank captains, Shoigu said.

He said the submariners acted heroically in a critical situation, evacuating a civilian from the compartment engulfed in fire and sacrificing their lives in an attempt to save the vessel, which was later towed to Severomorsk.

The details about the type of the vessel were not disclosed, including whether it had a nuclear propulsion system.