Over 20 Russian sailors were killed when a firefighting system went off during sea trials of a nuclear submarine in Russia's [Images] Pacific Fleet, Igor Dygalo, a senior navy commander was quoted as saying by the ITAR-TASS news agency on Sunday.

"The submarine is not damaged, its reactor works as normal, and radiation levels on the submarine are normal," Dygalo said.

The submarine was ordered to suspend the sea trials and return to port, Dygalo added.

Russia's Defence Minister Anatoly Serdyukov reported the incident to President Dmitry Medvedev who had ordered a "full and meticulous" investigation and "all possible aid and support to the victims' families," according to the report.