Fuel is being shipped from South Korea to Russia via sanctioned tankers as Moscow scrambles for supplies amid the war in Ukraine, according to an analysis of port records and ship-tracking data.

The analysis found that in July and August, over 176,000 tonnes of fuel, mostly diesel, was loaded at several South Korean ports onto seven tankers that made 14 voyages to Russia's far east. Three of the vessels were under sanctions by the UK and EU.

One tanker took on its cargo through a ship-to-ship transfer in South Korean waters, a method that can obscure a cargo's origin and allows ships to avoid calling at port terminals.

"It is shocking that EU-sanctioned tankers loaded fuel at South Korean ports and carried it to Russia," said Isaac Levi, an analyst at the Centre for Research on Energy and Clean Air (CREA), a Finnish think tank. "This undermines the sanctions regime and South Korea's support for Ukraine."

The largest carrier, the sanctioned tanker Layla, made two voyages from the south-eastern port of Ulsan to Vladivostok, departing on 19 July and 10 August and carrying 57,000 tonnes of diesel in total. On the first trip, it declared Vladivostok as its destination to South Korean authorities. On the second, it stated it was bound for Niigata, Japan.

Another sanctioned tanker, the Russian-flagged Astoria, also under Australian sanctions, loaded nearly 11,000 tonnes of fuel at Ulsan on 1 August, declaring Japan as its destination before sailing to Vladivostok.

A third sanctioned vessel, the Chongchon, never berthed at port. It took on approximately 26,000 tonnes of fuel from an unsanctioned tanker, the Celeste I, at an anchorage off the southern South Korean port of Yeosu in early August, then headed to Vladivostok after declaring Singapore as its destination. The owners and managers of the four tankers did not respond to requests for comment.

According to data from maritime analytics firm Kpler, buyers of the fuel included Rosneft, Russia's state-controlled oil company, which is under UK, US, EU and Australian sanctions, and NNK, a Russian oil group whose owner, Eduard Khudainatov, and one of its companies, NNK-Oil, are under UK sanctions.

Ukraine's military intelligence agency, the HUR, said Moscow has been forced to import fuel products to feed what it described as the "Russian war machine." Russia's capacity to refine oil has been severely disrupted by Ukrainian drone strikes. Its refinery throughput fell in June to its lowest level in more than 20 years, and diesel output is down nearly 30 percent compared to the previous year, according to the International Energy Agency.

In August, Russia imported 368,000 tonnes of oil products by sea, more than seven times the volume of the previous month and the highest monthly total since the full-scale invasion of Ukraine, according to Kpler data analysed by CREA. South Korea accounted for 112,000 tonnes, or 31 percent of that amount, second only to India.

Loadings at South Korean ports began to surge in July, just as South Korea's president, Lee Jae Myung, pledged $100 million for Ukraine's reconstruction at a NATO summit in Turkey.

While the fuel trade involves vessels sanctioned by some of Seoul's western partners, it does not appear to breach South Korean law. South Korea's own measures against Russia rely mainly on export controls on goods such as industrial machinery, electronics and vehicles.

Seoul has sent Ukraine humanitarian aid and non-lethal military supplies, including helmets, body armour and demining vehicles, and in 2023 pledged $2.3 billion in support, most of it in loans. Throughout the war, South Korea has ruled out sending weapons directly to Ukraine, in line with its practice of not supplying lethal arms to countries at war.

The shipments risk making Seoul appear a weak link in western efforts to pressure Russia, at a time when relations with Kyiv are already under strain over the transfer of two North Korean prisoners of war to South Korea, which Seoul said was intended to remain secret.

A 2025 trade association survey found that nearly 80 percent of South Korean firms that had stopped exporting to Russia since the war began were open to resuming trade. It is unclear who owned or sold the fuel, as South Korean port records do not disclose that information.