A fire on one of the Russian navy's deep-sea submersibles has killed 14 sailors, the Defence Ministry said, without giving the cause of the blaze or saying if there were survivors.
The ministry did not name the vessel that caught fire on Monday. Russian media reported it was Russia's most secret submarine, a nuclear-powered vessel designed for sensitive missions at great ocean depths.
President Vladimir Putin cancelled a scheduled appearance and summoned his defence minister to be briefed on the situation.
The Defence Ministry said in a statement that the fire broke out while the submersible was measuring sea depths in Russian territorial waters and the vessel is now at the Arctic port of Severomorsk, the main base of Russia's Northern Fleet. An investigation is under way.
The ministry said the fire was put out due to the self-sacrifice of crew members but did not say what is thought to have started it.
It also did not say how many people were on board the submersible or if any sailors survived the fire.
The ministry's statement said the submersible is intended for studying the seabed, but did not give its name or type. Russia's RBC online news outlet reported it was the nuclear-powered AS-12 Losharik.
The Losharik, which entered service in 2010, is the most advanced and most obscure Russian submarine. It is named after a Soviet-era cartoon character, a toy horse made of small spheres.
The name is apparently explained by the unique design of its internal hull, which is made of several titanium spheres capable of withstanding high pressure at great depths.
In 2012, the Losharik was involved in research intended to prove Russia's claim on the vast Arctic seabed. It collected samples from the depth of 2,500 metres, according to official statements at the time. Regular submarines can typically dive to depths of up to 600 metres.
Some observers speculated the Losharik was capable of going as deep as 6,000 metres, but the claims could not be independently confirmed.
Analysts suggested one of its possible missions could be disrupting communication cables on the ocean bed.
The Losharik is carried under the hull of a mother submarine, the nuclear-powered Orenburg.
Russian news reports said that while the Losharik officially belongs to the Northern Fleet, it answers directly to the Defence Ministry's Department for Deep-Sea Research, reflecting the high sensitivity of its missions.
The Russian navy also uses Priz-class and Bester-class deep water vehicles, which have hulls built of titanium and are capable of operating at a depth of 1,000 metres. They have a crew of two and are primarily intended for rescuing submariners in case of incidents.
Such vessels are transported to the area of operation by a carrier vessel and can operate autonomously for up to 120 hours.
The blaze is the deadliest Russian naval incident since 2008, when 20 died as a firefighting system was accidentally initiated while the Nerpa nuclear-powered submarine of Russia's Pacific Fleet was undergoing trials.
In the deadliest naval incident in post-Soviet Russia, the Kursk nuclear submarine exploded and sank on August 12 2000, during naval manoeuvres in the Barents Sea, killing all 118 crew members.