The revelation comes amid growing concerns over wide-ranging efforts by North Korea to evade U.N. Security Council sanctions over its nuclear weapons and missiles program.
A South Korean coast guard official said the 5,160-ton P-PIONEER has been prevented from leaving Busan port since October over allegations that it was used to supply diesel to North Korea through two ship-to-ship transfers in September 2017 in international waters in the East China Sea. He did not confirm how much was supplied.
South Korea's Yonhap news agency cited unnamed sources to report that the North Korean vessels on the receiving end were the Kum Un San and the Yu Son. The Kum Un San is believed to be one of North Korea's most active vessels involved in ship-to-ship transfers of petroleum products. The U.N. Security Council said in a report last month that an unnamed member state said that North Korea has been using approximately 23 of its tankers for the prohibited transfers, and that six tankers, including the Kum Un San, accounted for half of the transfers.
An official from Seoul's Foreign Ministry said it was the first time a South Korean ship was detained over allegations of violating U.N. sanctions against North Korea. South Korea is detaining three foreign vessels for illegally providing oil to North Korea or transporting North Korea-produced coal.
The Korea Coast Guard has sent the case to state prosecutors, who are now examining whether to indict the ship's captain and management company for violating laws governing maritime transport and exchanges with North Korea, the coast guard official said.
Both officials spoke on condition of anonymity, citing department rules preventing them from speaking to the press.
In its March report, the U.N. Security Council said North Korea was continuing to defy its resolutions through a "massive" increase in ship-to-ship transfers of petroleum products and coal. The report quoted one unnamed member state as saying North Korea obtained more than the cap of 500,000 barrels of refined petroleum products in 2018, but said another unnamed country questioned the figure.
U.N. sanctions against North Korea have strengthened significantly since 2016 as Pyongyang stepped up its weapons tests and now includes strict import restrictions on petroleum products and crude oil and export bans on coal and other minerals that had been key sources of foreign currency for the North.
South Korean prosecutors in December charged four people with illegally importing North Korean coal, saying they used falsified documents and transshipments at Russian ports to disguise the coal as Russian.