A fire aboard a secret Russian nuclear research vessel that killed 14 Russians may have been involved in secret operations by Moscow to support underwater drone operations.

Russia's Defense Ministry said in a statement that the fire broke out aboard a research submersible designed to study the seafloor and ocean bottom on July 1, killing a total of 14 "submariners" from smoke inhalation.

The location of the accident was not specified other than Russian territorial waters with reports from Russia saying the incident occurred in the Barents Sea, and the submarine described as the AS-12, nicknamed "Losharik," was towed to the Northern Fleet headquarters at Severomorsk.

According to state-run Russian news reports, the Losharik is designed for deep sea military missions, such as tapping into undersea communications cables.

The submarine reportedly is capable of diving to nearly 20,000 feet—deeper than other submarines can go because of the great pressures. It is powered by a single nuclear reactor and can travel up to 30 knots underwater.

The submarine is said to be constructed with ball-like components that give its hull strength. The nickname Losharik is from a Russian cartoon horse made up of balloon-like spheres.

In April, Moscow announced the launch of a very large nuclear-powered research submarine called the Belogorod that will carry nuclear drones.

A report in the state-run Izvestya in April 2017 said the Belogorod would also carry Losharik submersibles with a different designation than the AS-12. The report quoted Vadim Kozyulin, with the Academy of Military Sciences, as saying the Belogorod beginning in 2018 will carry "the AS-31 Losharik autonomous deep-sea station."

"It will transport and install on the seabed autonomous underwater nuclear modules for charging unmanned underwater vehicles," Kozyulin said. "The submarine will support the deployment of a global underwater environment monitoring system which the military is building at the bottom of the Arctic seas."

Mark Schneider, a former Pentagon nuclear policymaker, said the accident took place on a very deep-diving nuclear powered submarine. The BBC reported that among the 14 dead were 7 senior navy captains and 2 "Heroes of Russia."

"This obviously means that this is a very high priority military research program," Schneider said. "I doubt they would put this level of effort on pure science research. Russian submarine safety has been bad since the Soviet period."

The pro-Kremlin news site Pravda.ru quoted a military source as saying surviving crew members were able to put out the fire and the crew had been engaged in "biometric measurements."

The submarine was said to have taken part in undersea intelligence operations in the arctic and Sea of Japan.

Sputnik news service, a state-controlled outlet, quoted Russian president Vladimir Putin as saying seven captains of the first rank and two Heroes of Russia died in the incident. The Russian leader called the deaths "a great loss."

Radio Free Europe/Radio Liberty reported that the submarine, according to a naval blogger, was engaged in deep-diving special missions under the command of Russian Navy research unit Main Directorate Deep Sea Research (GUGI).

According to RFE/RL, GUGI conducts undersea special operations with a fleet of submarines designed for covert operation. The organization is under the Russian GRU military intelligence service and not the Russian Navy.

A photo of the submarine was captured by the Russian-language version of the magazine Top Gear.

The latest accident follows a 2008 incident involving a Freon gas leak on a nuclear-powered submarine called the Nerpa that killed 20 and injured 21.

Russia's biggest submarine disaster took place in August 2000 when the nuclear-powered submarine Kursk sank in the Barents Sea as a result of two explosions. The sub sank and killed all 118 sailors on board.

The Losharik is said to have been first developed in the 1980s and first deployed in the late 1990s. It is believed to carry a crew of 25.

In addition to using its own power, the Losharik can be towed underneath other larger submarines.

The submarine appears to be part of Russian efforts to extend sovereignty over areas of the arctic.

The Pentagon has expressed concerns about Russian military encroachment into the arctic. "Russia has …. threatened to use force against vessels that fail to abide by Russian regulations," an arctic strategy report made public earlier this year states.

"The arctic is strategic terrain as a potential vector for an attack on the U.S. homeland," the report said. "China and Russia pose discrete and different challenges in their respective theaters, but both are also pursuing activities and capabilities in the arctic that may present risks to the homeland."

The Washington Times reported that the secret Losharik submarine may be involved in Moscow's effort to tap into or sever some of the 550,000 miles of underwater fiber-optic cables that span the Atlantic and Arctic sea lanes. The newspaper stated that Losharik is likely more than a research vessel and conducts reconnaissance missions for Russian intelligence and military special operations. The NATO code name for the vessel is NORSUB-5.

The Kremlin said details of the accident would not be made public but denied claims that an explosion took place on board the submarine.