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Fourteen Russian navy crew killed in a fire on a nuclear submarine on Monday have been buried, amid continuing secrecy over their mission.

They were buried in the Serafimovskoye Cemetery of the country's second city, St Petersburg, under tight security.

All of them were given posthumous honours, and four - all captains - were awarded the nation's highest honour, Hero of the Russian Federation.

Russia has not revealed any details about their mission.

The submarine has not been officially identified, and what it was doing is considered a state secret.

The Kremlin says the crew contained a fire that broke out aboard the vessel, saving the nuclear power reactor, and that they died from inhaling toxic smoke.

But so great is the secrecy around the mission that the gates of the cemetery were guarded by military police, and one official told AFP news agency the identities of those attending were mostly kept secret.

Dmitry Razmyslov, a friend of dead crew member Dmitry Solovyev, told AFP the crewman had left behind a pregnant wife. "Dmitry was attentive and rigorous, he carried out orders without fault," he said.

"It's hard to say what happened but he preferred to do what he had to do despite his family situation."

The vessel was based in Severomorsk in northern Russia.

Despite the official secrecy, Russian media have widely reported that the craft was a S-12 or AS-31, nicknamed "Losharik", which had been exploring the Arctic seabed in the Barents Sea.

The crew have been linked in media reports to a top-secret Russian military branch known as the Chief Directorate for Deep Water Research, or GUGI.

A military analyst for Russian newspaper Novaya Gazeta speculated the vessel might have been involved in testing new secret equipment or examining devices left on the sea floor by the United States.

Russia is known to be staking a territorial claim to the Arctic, on the basis that its continental shelf extends north under the ocean.

But competing claims by other nations - Canada, the US, Norway and Denmark - are being examined by the United Nations.