Moscow: In Russia's worst naval accident in nearly a decade, at least 20 people, including three sailors were killed and 21 injured in a gas leakage during a sea trial of a brand-new nuclear-powered Nerpa submarine that was to be leased to India later this year.

"Autopsy results confirm that the people died as a result of inhaling freon gas released when the fire-extinguishing system was activated in one of the submarines compartments" on Saturday night, a spokesman for the federal probe panel said.

Freon is a cooling agent used in automatic fire-extinguishing systems. Due to accidental activation, the compartment was "hermetically" sealed and technicians inside it were suffocated, while in the usual condition the crew uses individual gas masks, Vesti channel reported.

Russian President Dmitry Medvedev has ordered a high-level probe into the accident aboard K-152 Nerpa, the worst since the 2000 Kursk nuclear submarine mishap.

Naval spokesman Capt Igor Dygalo said there was no radiation leak. The sub has since returned to the base from the Pacific Ocean, with its reactor working in the normal mode

There were 208 people -- 81 of them servicemen -- on board the vessel at the time of accident.

"According to preliminary information, the incident killed 20 people -- three sailors and 17 civilian technicians who were aboard the submarine while 21 received injuries of various gravity," spokesman for the investigation Vladimir Markin said.

NTV channel reported that the nuclear submarine was to be commissioned later this year and an Indian crew was expected to arrive to take the charge of the vessel.

Christened as INS Chakra, the submarine was to be leased to India for ten years.

Capt Dygalo said the vessel has not been damaged. Further tests to be conducted on it have since been cancelled on the order of the Navy commander-in-chief.

Medvedev has instructed the Defence Ministry to conduct a thorough and comprehensive investigation into the accident and provide the necessary assistance to the victims families.

The incident is Russias worst naval accident since 2000 when another nuclear submarine, Kursk, alongwith the 118 crew, had sank in the Barents Sea after an explosion.

Later in August 2003,nine of the ten sailors on board a K-159 submarine died when it sank in the Barents Sea.

Source : DNA