Vladimir Putin has admitted that the fire that killed 14 sailors on Monday happened on a nuclear submarine, although the defence ministry said the reactor was not damaged. Ministry head Sergei Shoigu met with Mr Putin on Thursday upon his return from the Arctic naval town of Severomorsk, where the stricken submersible was taken. During their conversation, Mr Putin asked him about the nuclear power unit on board, according to a Kremlin readout. Moscow had not previously confirmed the presence of a reactor and still has not said what kind of submarine was involved, arguing that these are state secrets. Mr Shoigu told Mr Putin that "the nuclear power unit has been sealed off and all personnel have been removed". He also said the cause of the disaster was a "fire in the battery compartment which then spread". The blaze happened on Monday but was only announced the next day by Russian authorities, who have refused to give many details about the incident. Russia rescue personnel return from a dive in a mini submarine to the Kursk on the sea bed Credit: RU-RTR Russian Television via AP It is believed that the fire occurred on an AS-31 deep-water research submarine in the Barents Sea. Nicknamed "Losharik" after a Soviet cartoon horse, it is operated by a navy division responsible for reconnaissance and intelligence gathering and can reportedly dive to depths of 10,000 feet. The submarine is reportedly able to cut underwater cables that carry much of the world's internet traffic. All 14 of the sailors who died were officers, leading to speculation that it was on a top-secret mission. The defence ministry said they were performing "bathymetric measurements" of the ocean floor. One sailor put a civilian in a compartment and shut the hatch to return to fighting the blaze, it said. Arctic neighbour Norway said it had been informed of a gas explosion on the submarine, but the Russian defence ministry denied this. Norwegian authorities said they had not detected abnormal levels of radiation in the Barents Sea