At least 20 people have died in an incident involving the failure of a fire extinguishing system on a Russian nuclear submarine, local media report.
Russian Pacific Fleet spokesman Igor Dygalo said both sailors and shipyard workers died in the incident, which occurred during sea trials.
He said the submarine itself had not been damaged and there had been no radiation leaks.
Military prosecutors are investigating the incident.
The submarine, whose name and class have not been revealed, has been ordered to suspend sea trials and return to port in the far eastern Primorye territory, Capt Dygalo said.
The sinking of the Kursk was Russia's worst submarine disaster
"I declare with full responsibility that the reactor compartment on the nuclear-powered submarine is working normally and the radiation background is normal," he said, quoted by Itar-Tass news agency.
There were 208 people on board at the time, 81 of whom were servicemen.
Twenty-one injured people have been evacuated from the submarine, sources at the fleet said.
Reports say the incident occurred in the nose of the vessel. The nuclear reactor, which is in the stern, was not affected.
Russian President Dmitry Medvedev is being kept fully informed about the incident, his press service said.
Deputy Defence Minister Alexander Kolmakov and Navy Commander-in-Chief Vladimir Vysotsky are flying to the scene of the incident.
Russia's worst submarine disaster happened in August 2000, when the nuclear-powered Kursk sank in the Barents Sea. All 118 people on board died.