Four ships - three North Korean vessels and a Palau-flagged oil tanker - were blocked from global ports by the UN Security Council on Thursday on suspicion of carrying or transporting goods banned by the sanctions.
South Korea is holding a second ship on suspicion of supplying oil to North Korea in violation of worldwide sanctions meant to punish Kim Jong-un's regime for its nuclear weapons and missile programmes.
The 5,100-tonne KOTI was detained two weeks ago at the South's port of Pyeongtaek-Dangjin and its crew are being investigated over the alleged ship-to-ship transfers, Yonhap news agency cited local maritime officials as saying.
Authorities are now investigating the matter and won't disclose further details, Yonhap reported, adding the ship's crew was mostly made up of citizens from China and Myanmar, just like the Hong Kong-flagged tanker Lighthouse Winmore that was seized on Friday.
Results of the investigation will be reported to the UN Security Council's sanctions committee, foreign ministry officials said.
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The resolution adopted by the council included sharply lower limits on North Korea's refined oil imports, the return home of all North Koreans working overseas within 24 months, and a crackdown on ships smuggling banned items including coal and oil to and from the country. It is unclear when exactly KOTI was seized, but customs officials decided on December 21 to deny the ship permission to leave the port, a local maritime official told South Korean news agency Yonhap.
The vessel is suspected of transferring 600 tonnes of oil to the North Korean tanker Sam Jong 2 in worldwide waters on October 19.
It did not include even harsher measures sought by Washington that would ban all oil imports and freeze worldwide assets of the government and its leader Kim Jong Un. The KOTI does not appear to be on the list.
The seizure was the second to be revealed by South Korea within a few days, as the United Nations steps up efforts to squeeze essential oil supplies to the reclusive North following its nuclear or ballistic missile tests. It is Panamanian flagged. He could not clarify whether they belonged to mainland China or Hong Kong.