MOSCOW — At least 14 Russian military personnel, including high-ranking officers, died from smoke inhalation after a fire broke out on a deep-sea research vessel, the Defense Ministry said Tuesday, leaving questions over the role of the highly secretive craft.

The fire took place Monday while crew on the craft were conducting topographic measurements of the seabed near Russia’s Severomorsk base in the Barents Sea, the ministry said. 

The vessel, however, is linked to the Defense Ministry’s unit for underwater intelligence, which is tasked with sensitive missions such as mapping and monitoring ocean depths, Russian media reports said.

President Vladi­mir Putin, who called an emergency meeting with Defense Minister Sergei Shoigu, said at least half of those killed on the vessel had the rank of captain. Two of the dead had received the Hero of the Russian Federation award, one of the country’s highest honors.

Shoigu said an investigation was underway.

Russian media outlets, citing military sources, identified the vessel as a nuclear-powered AS-12 Losharik submersible. The craft was recovered and returned to base.

The Defense Ministry did not indicate how many crew members were aboard when the fire occurred, but the vessel can hold up to 25 people.

Differing from a submarine, a submersible is usually smaller and operates with some form of support. A submarine is completely autonomous. The Losharik is carried in the hull of a submarine, the nuclear-powered Orenburg, and descends to far-deeper depths.

Russian media said the Losharik can reach more than 18,000 feet below the surface, but military officials have not publicly given the vessel's range of operations.

"This is not an ordinary ship," Putin said.

NATO has expressed concern over an increase in apparent Russian undersea activity around data ­cables in the North Atlantic. The military alliance worries that vessels such as the Losharik are working the ocean floor, allowing Russia to potentially sever or tap the cables. 

There was no immediate statement from NATO on Monday’s undersea incident. The Barents Sea is north of the Russian and Norwegian coasts.

Norway’s Radiation and Nuclear Safety Authority said there were no readings of radiation spikes in the Barents Sea after the submersible fire, the Reuters news agency reported.

Four years ago, Pentagon officials raised an alarm over the Russian spy ship Yantar, which is equipped with deep-sea submersibles, after it cruised near the East Coast on its way to Cuba. The Pentagon worried the Russian ship was too close to major cables near the U.S. military base at Guantanamo Bay. 

In 2012, the Losharik — named for a Soviet-era cartoon horse — was involved in reconnaissance missions that sought to bolster Russia’s claim to vast stretches of the Arctic seabed.

In recent years, Russia has moved to modernize its once-decrepit fleet of Soviet-era submarines, coinciding with a new era of confrontation with the West as Moscow pursues military campaigns in Syria and alongside pro-Moscow separatists in Ukraine. 

The plans are part of Putin’s broader push for new defense hardware, including naval drones and hypersonic missiles. 

Monday’s death toll is the largest for an underwater vessel in Russia since 2008, when 20 sailors died during a failed sea trial on a submarine. 

Russia is still haunted by the sinking of the Kursk nuclear submarine at the start of Putin’s first year in power in 2000, when 118 sailors died in the Barents Sea. His slow initial response was widely seen as a public relations disaster for the then-new leader. 

Brian Murphy in Washington contributed to this report.