Russian tankers have reportedly and illicitly supplied fuel to North Korea on at least three occasions in recent months, by transferring cargoes at sea.
The sales of oil or oil products from Russia, the world's second biggest oil exporter and a veto-wielding member of the United Nations Security Council, breach UN sanctions, say two senior Western European security sources.
The transfers in October and November indicate that smuggling from Russia to North Korea has evolved to loading cargoes at sea, since Reuters reported in September that North Korean ships were sailing directly from Russia to their homeland.
"Russian vessels have made ship-to-ship transfers of petrochemicals to North Korean vessels on several occasions this year in breach of sanctions," the first security source, who spoke on condition of anonymity, told Reuters.
A second source, who independently confirmed the existence of the Russian ship-to-ship fuel trade with North Korea, said there was no evidence of Russian state involvement in the latest transfers.
"There is no evidence that this is backed by the Russian state, but these Russian vessels are giving a lifeline to the North Koreans," said the second European security source said.
The two security sources cited naval intelligence and satellite imagery of the vessels operating out of Russian far eastern ports on the Pacific, but insisted further details were classified.
Russia's Foreign Ministry and the Russian Customs Service both declined to comment, when asked if Russian ships had supplied fuel to North Korean vessels.
The owner of one ship accused of smuggling oil to North Korea denied any such activity.
The latest report came as China, responding on Friday (local time) to criticism from US President Donald Trump, denied it had illicitly shipped oil products to North Korea.