The fire broke out at 8:30 p.m. on Monday, RBC cited its source as saying, nearly a day before the ministry released the news.

The incident took place on the AS-31 deep-sea nuclear submarine nicknamed Losharik, an unnamed source in the security forces told the RBC news website. Launched in 2003, Losharik was designed for research, rescue and special military operations and can hold up to 25 crew members.

Fourteen submariners on board a Russian Defense Ministry research vessel were killed in a fire while carrying out a survey of the sea floor off Russia's Arctic coast, the ministry was quoted as saying by Russian news agencies.

"On July 1 in Russian territorial waters a fire broke out on board a deep-water scientific research vessel that was studying the marine environment of the world ocean on behalf of the Russian navy," Interfax cited a ministry statement as saying.

"Fourteen submariners died as the result of smoke inhalation ... Work is underway to establish the cause of the incident. The investigation is being conducted by the commander-in-chief of the navy."

The fire has been extinguished and the submarine is now at the Russian Northern Fleet's base in Severomorsk on the Barents Sea, Interfax quoted the ministry as saying. The statement as cited by the agencies did not identify the type or model of the underwater vessel.

This is the largest accident to take place on a Defense Ministry submersible since 2008, when a freon gas leak on the nuclear-powered submarine Nerpa killed 20 and injured 21.

In August 2000, the Russian nuclear-powered submarine Kursk sank to the floor of Barents Sea after two explosions in its bow, killing all 118 men aboard.

That accident, soon after President Vladimir Putin took office, focused official attention on the state of the military and its hardware, which had been subject to underfunding and neglect after the collapse of the Soviet Union.

Since then, Putin has overseen a massive increase in military funding that has allowed the armed forces to renew their equipment and improve training and morale.

However, accidents have continued to happen as the military, used by the Kremlin to project its growing international muscle, has ramped up its activities and extended into new theatres of operation.

In December 2016, a Russian military plane carrying 92 people, including dozens of Red Army Choir singers, crashed into the Black Sea en route to Syria where Russian forces are deployed. Everyone on board was killed.

Reuters contributed reporting to this article.