An accident with a fire-extinguishing system on a Russian Navy nuclear submarine killed 20 people during tests at sea in the Pacific late Saturday night, Navy officials said.

The news came amid reports in the Indian media that the vessel in question had been undergoing tests prior to being leased to the Indian navy.

Three naval personnel and 17 civilian specialists died after inhaling chemical gases released when the firefighting system was erroneously activated as the submarine sailed through the Sea of Japan, off Russia's east coast, Navy spokesman Igor Dygalo said Sunday.

He said the nuclear reactor that powered the submarine was working normally and that the radiation level on board was within allowable limits.

It was unclear Sunday what had triggered the system. Dygalo said a commission of experts and investigators from the General Prosecutor's Office would examine the submarine before reporting on the cause of the accident.

Twenty-one people were hospitalized after inhaling the chemicals, Dygalo said, adding that they were all out of danger.

Yuri Maltsev / Reuters What is believed to be an Akula-class sub seen at July training in Vladivostok.

The submarine returned to its temporary Bolshoy Kamen port in the Primorye region, on Sunday morning, about 10 hours after the accident.

Dygalo said 208 people were aboard, including 81 sailors, when the accident occurred. He did not provide a name or any other details for the submarine, saying the vessel had yet to be commissioned.

Vladimir Markin, spokesman for the Investigative Committee under the Office of the General Prosecutor, said Sunday that medical examinations indicated that the victims had inhaled Freon, a gas most commonly known for its use in refrigerators and air conditioners.

He said employees at the shipyard that assembled the submarine were among those killed, Interfax reported. Markin did not name the shipyard.

Pacific Fleet military prosecutors opened an investigation Sunday into possible violations of naval regulations that could have led to the deaths.

Worst Submarine Accidents   The following is a chronology of major accidents involving nuclear submarines since 1968: May-June 1968: U.S. Navy submarine Scorpion sinks with 99 men aboard east of Norfolk, Virginia. March 4, 1970: French submarine Eurydice sinks with 57 crewmen off St. Tropez in the Mediterranean Sea. April 12, 1970: A Soviet November class nuclear-powered attack submarine sinks with 88 crew members in the Atlantic Ocean off Spain. Aug 10, 1985: An explosion on a Soviet submarine at Chazma Bay, on the Pacific coast near Vladivostok, sends the lid of the reactor flying hundreds of meters with no trace of the 10 people working in the reactor. April 7, 1989: A Soviet Mike class attack submarine sinks off northern Norway with the loss of 42 lives after a fire. Aug 12, 2000: The Russian Oscar-II class submarine Kursk with 118 crew members sinks to the bottom of the Barents Sea after two explosions on board. Feb. 9, 2001: The Greenville, a U.S. Navy submarine based at Pearl Harbor, sinks a Japanese trawler after colliding with it, killing nine civilians aboard the fishing boat. January 2005: One crewman dies after the U.S. nuclear submarine San Francisco runs aground off Guam in the Pacific Ocean. Another 23 crewmen were injured. Sept. 6, 2006: Fire on board the Viktor-3 class Russian submarine St. Daniel of Moscow, moored near the Finnish border, kills two crew members. March 21, 2007: Two British soldiers are killed and another injured on the Tireless nuclear submarine during an exercise in the Arctic. Nov 8, 2008: More than 20 people killed and another 21 injured on a Russian nuclear submarine in the Pacific. The Russian Navy did not disclose the name or exact location of the submarine. Source: Reuters

President Dmitry Medvedev was informed about the accident early Sunday and called Defense Minister Anatoly Serdyukov for further updates and to order that assistance be provided for the victims' families. Medvedev also ordered General Prosecutor Yuri Chaika to conduct a thorough investigation of the accident.

The accident was the Navy's deadliest since an explosion in 2000 sank another nuclear submarine, the Kursk, in the Barents Sea, killing all 118 crewmen on board.

"Such deadly accidents, but of a smaller size, that occur from time to time on nuclear-powered vessels have already become a norm for the Russian Navy," said Konstantin Makiyenko, a defense industry analyst with the Center for Analysis of Strategies and Technologies, a think tank.

Both Makiyenko and Alexander Khramchikhin, an expert at the Institute of Political and Military Analysis, both said neglect in following safety regulations was the likely cause for the accident.

"I believe the civilians from the plant were likely to blame," Khramchikhin said. "They were in charge of the submarine at that moment."

In late October, Russian media, including state-controlled RIA-Novosti, reported that a new submarine, the Nerpa, a Project 971 Shchuka-B attack submarine (NATO classification Akula-class) built at the Amur Shipbuilding plant, had begun its sea tests. A source at the plant confirmed the report Sunday, RIA-Novosti reported.

The source said plant engineers and builders were on board when the accident occurred. Construction of the Nerpa began in 1991, but was stalled through the 1990s for lack of financing.

The Indian media has long reported, and continued to report Sunday, that construction of the Nerpa was resumed with Indian funding after Russia agreed to lease the submarine to the Indian navy for $650 million on its completion, citing top Indian military officials. The submarine was to be named Chakra-2 and join the Indian navy in the second half of 2009, the reports said.

Serdyukov said in late September that Russia does not export nuclear submarines.

India, Russia's biggest arms customer over the last two decades and a traditional ally in Asia dating to Soviet times, leased a nuclear submarine from Moscow from 1988 to 1991.

Khramchikhin maintained that the submarine that suffered the accident was, indeed, the vessel earmarked for the Indian navy.

"There is no other," he said.

One defense analyst, speaking on condition of anonymity because of the sensitivity of the issue, said it was unlikely the accident would scare India away from the deal, as none of the submarine's most critical systems -- weapons and propulsion -- were revealed as faulty.