Russian tankers have supplied fuel to North Korea on at least three occasions in recent months by transferring cargoes at sea, breaching United Nations sanctions, sources told Reuters.
The Panamanian-flagged vessel, known as KOTI, is now under investigation by South Korea.
The ship can carry 5,100 tons of oil and has a crew mostly from China and Myanmar, Yonhap News Agency reported, adding that South Korea's intelligence and customs officials are conducting a joint probe into the vessel.
Speaking on condition of anonymity, one western security source said: "Russian vessels have made ship-to-ship transfers of petrochemicals to North Korean vessels on several occasions this year in breach of sanctions.
China will continue to participate in the work of the relevant Security Council sanctions committee on this principle", it said in a short statement, without elaborating. It did say resolutions by the United Nations Security Council have imposed limits on North Korea's refined oil imports but haven't banned them altogether.
Beijing on Friday denied selling oil products to North Korea even though the Lighthouse Winmore is owned by a company registered in Guangzhou, while the vessel sails under a Hong Kong-flag.
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The ship, seized at Yeosu Port on November 24, is suspected of transferring 600 tons of refined petroleum to a North Korean ship named Samjong No. 2.
The move comes after months of attempts by Trump to pally up with Russian Federation and China in a bid to win their support over North Korea.
The Hong Kong government said in a statement Friday it had noted media reports that the Lighthouse Winmore had been seized.
Last week the UN Security Council unanimously adopted new sanctions on North Korea in response to Pyongyang's November 29 ballistic missile test, seeking to further strangle its energy supplies and tighten restrictions on smuggling and the use of North Korean workers overseas.
China denied the allegations, saying reports that it sold oil to North Korea did "not accord with the facts". The KOTI does not seem to be included on the list.
CNN's Taehoon Lee reported from in Seoul, Lauren Said-Moorhouse wrote from London while Yuli Yang and James Griffiths contributed from Hong Kong.