Officials seized the KOTI, a Panamanian-flagged vessel, after it was suspected of illegally selling oil to North Korea, maritime authorities told South Korea's Yonhap news agency.
South Korean officials are holding the 5100-tonne, Panama flagged tanker called KOTI in Pyeongtaek-Dangjin after deciding not to let release the vessel on December 21 to investigate further.
South Korean authorities have seized a Hong Kong-flagged vessel believed to have secretly transferred oil to a North Korean vessel in worldwide waters in a ship-to-ship transfer prohibited by the U.N. Security Council, local media reported Friday.
The Russian Foreign Ministry on Saturday denied claims that United Nations sanctions against North Korea had been breached by Russian tankers transferring fuel to North Korean tankers at sea.
"We are closely monitoring North Korea's attempts to evade United Nations sanctions".
Four ships - three North Korean vessels and a Palau-flagged oil tanker - were blocked from worldwide ports by the UN Security Council on Thursday over suspicions of carrying or transporting goods banned by sanctions targeting Pyongyang's weapons ambitions, according to the final list adopted by the world body.
The ship had headed towards its purported destination in Taiwan, but on reaching global waters transferred some of its 600 tonne cargo to the Sam Jong 2 and three other North Korean vessels, the South claimed. The Lighthouse Winmore was seized upon re-entering Yeosu on November 24, foreign ministry officials told CNN.
China denied Friday that it was supplying oil to North Korea, USA Today reported.
The US Treasury Department released satellite imagery in November of two ships allegedly performing an illegal ship-to-ship transfer in worldwide waters on the same day.
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 KOTI's ties to China also seem clear, the tanker is run by companies operating out of Hong Kong and Dalian, two well-known hubs for North Korean sanctions evasion.
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 UN Security Council has imposed three sets of sanctions on the North this year to try to stop its nuclear and missile tests.