Two security sources have told news agency Reuters (ROY-ters), they have cited naval intelligence and satellite imagery of the vessels operating out of Russian Far Eastern ports on the Pacific.
The alleged oil transfers from Russia to North Korea, break United Nations sanctions and provide an economic lifeline to the secretive Communist state.
But Russia's Foreign Ministry and the Russian Customs Service have both declined to comment on the matter and the owner of one ship accused of smuggling oil to North Korea has denied any such activity.
It comes as the crew of a Hong Kong-registered ship have been detained for questioning in South Korea since their tanker was impounded in November for transferring oil to a North Korean vessel.
