MOSCOW – An accident aboard a nuclear-powered Russian navy submarine doing a test run in the Pacific Ocean on Saturday killed more than 20 people, the navy said.

The nuclear reactor aboard the submarine was operating normally and radiation levels were normal, navy spokesman Capt. Igor Dygalo said on Sunday.

The accident occurred when a fire-extinguishing system went into operation in error aboard the submarine, Dygalo said. He said there were 208 people aboard, including 81 servicemen. The dead included sailors and shipbuilders.

The submarine was heading back to shore on its own power, and 21 people injured in the accident were evacuated to a ship that was escorting the sub.

Dygalo said the destroyer Admiral Tributs and a rescue vessel were accompanying the submarine.

The state-run RIA-Novosti news agency cited an unnamed official in Russia's Pacific Fleet as saying the accident occurred toward the bow of the submarine and that there was no threat to the nuclear reactor, closer to the center.

Dygalo would not name the submarine or say exactly where the accident took place.

RIA-Novosti cited an unnamed official at the Amur Shipbuilding Factory as saying the sub was built there, is called the Nerpa and was on a test run in the Sea of Japan when the accident occurred.

The official said the sub was heading for a base in the Primorye region, which includes the port city of Vladivostok.

It was unclear what activated the fire-extinguishing system or how the victims were killed. RIA-Novosti cited an unnamed shipbuilding expert as saying fire systems on Russia's nuclear-powered submarines usually use foam and chemicals.

According to RIA-Novosti, testing on the submarine began last month and it submerged for the first time last week.

First Deputy Defense Minister Alexander Kolmakov and navy chief Adm. Vladimir Vysotsky were heading for Russia's Pacific Coast in the wake of the accident, Dygalo said.

The Kremlin said President Dmitry Medvedev had been informed about the accident immediately by his defense minister and was receiving frequent updates, Russian news agencies reported. Medvedev ordered a thorough investigation.

Russian Prime Minister Vladimir Putin was criticized for his slow response to the Kursk disaster, which marred the first of his eight years as president.

Russia's navy has been plagued by deadly accidents, including the explosions that sank the nuclear-powered submarine Kursk in the Barents Sea in 2000, killing all 118 seamen aboard. A submarine that was being taken out of service sank in the Barents Sea in 2003, killing 11 people.

Saturday's accident came as the Kremlin seeks to restore Russia's naval reputation, part of a drive to show off the country's military clout amid strained ties with the West. A naval squadron is headed to Venezuela for joint exercises this month in a show of force near U.S. waters.