LONDON/MOSCOW - Russian tankers have supplied fuel to North Korea on at least three occasions in recent months by transferring cargoes at sea, according to two senior Western European security sources, providing an economic lifeline to the secretive Communist state.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 U.N. sanctions, the security sources said.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," 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 declined to disclose further details to Reuters, saying it was classified.Russia's Foreign Ministry and the Russian Customs Service both declined to comment when asked on Wednesday 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 to criticism from US President Donald Trump, denied it had illicitly shipped oil products to North Korea.North Korea relies on imported fuel to keep its struggling economy functioning. It also requires oil for its intercontinental ballistic missile and nuclear program that the United States says threatens the peace in Asia."The vessels are smuggling Russian fuel from Russian Far Eastern ports to North Korea," said the first security source, who spoke on condition of anonymity.Reuters was unable to independently verify that the vessels had transferred fuel to North Korean vessels, whether the Russian state knew about the sales or how many Russian vessels were involved in the transfers. It was also unclear how much fuel may have been smuggled.Ship satellite positioning data consulted by Reuters and available on Reuters Eikon shows unusual movements by some of the Russian vessels named by the security sources including switching off the transponders which give a precise location.The security sources said the Russian-flagged tanker Vityaz was one vessel that had transferred fuel to North Korean vessels.