A nuclear-powered submarine belonging to the Russian Navy was involved in a fatal accident in which at least 20 shipbuilders and servicemen were killed. The accident occurred on Saturday when the submarine was conducting sea trials in the Sea of Japan. The cause of the accident was a faulty activation of the fire-extinguishing system aboard the vessel.

More than 20 people were injured in the incident, which was the worst incident involving a Russian submarine since the explosion of Kursk in 2000 when 118 were killed.

However, the submarine returned to its base at Bolshoi Kamen near Vladivostok using its own power. The nuclear reactor aboard the submarine wasn’t damaged and the radiation levels are reportedly “normal,” a Russian Navy spokesman said.

"During sea trials of a nuclear-powered submarine of the Pacific Fleet the firefighting system went off unsanctioned, killing over 20 people, including servicemen and workers," said Captain Igor Dygalo, the navy's spokesman.

State RIA Novosti news agency reported that the sea trials were being conducted in anticipation of a planned delivery of the submarine to India.

According to a source with the Pacific Fleet's hospital in Vladivostok, several injured people from the Russian submarine were suffering of various degrees of poisoning, AFP reported. There were about 208 people on the submarine, most of them technicians and specialists.

A “full and meticulous” investigation has been ordered by President Dmitry Medvedev. The most probable cause of the incident seems to be human error.

The submarine and a rescue vessel reached the Bolshoi Kamen port on Sunday.