Moscow  - A gas blast onboard a cargo ship docked for repairs in Russia's European exclave of Kaliningrad killed three workers with seven more missing, Russian news agencies reported Friday.

Investigators blamed the explosion Thursday afternoon on a "violation of safety rules during repair work on the fuel tank," a source close to the inquiry was quoted by RIA-Novosti as saying.

Russian television showed a fire and smoke billowing from the freighter's deck in a military dockyard off the Baltic Sea.

Media reported three people dead and seven still missing Friday morning. Three other workers were reported injured in the blaze.

Russian navy spokesman Igor Dygalo said Friday that the cargo ship belonged to a private St Petersburg company who had leased the docks in the closed military port town near the city of Kaliningrad. (dpa)