South Korea has said it will take legal action if an investigation finds that fuel shipments from its ports to Russia broke domestic law, responding to allegations that have deepened a diplomatic rift with Ukraine. In a joint statement on Thursday, the foreign and trade ministries and the customs service said they were checking additional information and would "act in accordance with the law" if violations were established.

The allegations stem from a Guardian analysis of port records and ship-tracking data. It found that more than 176,000 tonnes of fuel, mostly diesel, were loaded at several South Korean ports in July and August onto seven tankers that made 14 voyages to Russia's far east. Three of the tankers are under UK and EU sanctions, and one took on about 26,000 tonnes in a ship-to-ship transfer off the southern port of Yeosu. Several vessels gave destinations such as Japan or Singapore before sailing on to Vladivostok. Vladyslav Vlasiuk, an adviser to Ukrainian President Volodymyr Zelenskyy on sanctions policy, said the shipments contrasted sharply with Seoul's sanctions policy and helped ease Russian fuel shortages. Ukraine's foreign ministry said it had raised the matter through closed diplomatic channels and that Kyiv had its own data.

Seoul rejects the suggestion that it has broken its rules. The government says it has not confirmed that South Korean-origin fuel was sent to Russia, and that official trade statistics show no exports of diesel, jet fuel or gasoline to Russia through customs clearance from 2025 through August 2026. It also stresses that commercial petroleum products are not covered by its export controls on Russia, which focus on strategic items and 1,402 non-strategic goods that could have military uses, such as industrial machinery, electronics and vehicles. Loading such fuel at its ports, officials say, is therefore not in itself illegal. They added that a vessel's change of course from its declared destination does not automatically prove false declaration, but said they would examine the tankers' actual routes and the reasons for any divergence. It remains unclear who owned or sold the fuel. The presidential office called Vlasiuk's remarks "contrary to fact and inappropriate" and urged Kyiv not to make public comments that could damage South Korea's international credibility.

The dispute comes as relations between the two countries are already strained. Seoul accused Zelenskyy of unilaterally disclosing to the UN General Assembly that Ukraine had transferred two captured North Korean soldiers to South Korea, and demanded an apology, saying a non-disclosure agreement had been breached. Kyiv says no such agreement existed. North Korea, which remains technically at war with the South because the 1950–1953 Korean War ended in an armistice, signed a mutual defence treaty with Moscow in 2024 and, by Ukrainian and South Korean estimates, sent 14,000 to 15,000 troops to support Russia's war.

The issue matters because Russia's refining capacity has been badly hit by Ukrainian drone strikes. The International Energy Agency says refinery throughput fell in June to its lowest level in more than 20 years and diesel output is down nearly 30% on last year. An analyst at the Centre for Research on Energy and Clean Air said the trade undermines the sanctions regime and South Korea's support for Ukraine. Seoul says it will keep enforcing export controls and strengthen efforts to prevent circumvention.