How the Fuel Reached Russia
The routes were far from straightforward. One vessel picked up its cargo from another ship at sea instead of loading at a port, while several others listed Japan or Singapore as their destination before turning toward Russia. Who owned or sold the fuel is still unclear. Data from analytics firm Kpler shows, however, that buyers included the state-controlled company Rosneft, which is under UK, US, EU and Australian sanctions.

Isaac Levi, an analyst at the Finnish think tank Centre for Research on Energy and Clean Air (CREA), said the trade weakens the sanctions regime and undercuts South Korea's support for Ukraine.

Why Russia Needs the Fuel
There is a clear reason behind the demand. Ukrainian drone strikes have hit Russian oil refining hard. The International Energy Agency says refinery throughput dropped in June to its lowest level in more than 20 years, and diesel output fell by nearly 30% compared with a year earlier.

Imports have climbed to fill the gap. Russia brought in 368,000 tons of oil products by sea in August, the highest level since the full-scale invasion began. According to CREA's analysis of Kpler data, South Korea supplied 112,000 tons of that, a figure second only to India.

Shadow Fleets and Seoul's Position
Much of Russia's oil moves through a so-called shadow fleet, often made up of ageing tankers with murky ownership and insurance. These ships help get around Western sanctions and the price cap on Russian crude. The EU has sanctioned more than 670 such vessels and Britain more than 600, which bars them from ports and Western maritime services.

Ship-to-ship transfers, where cargo is pumped between vessels at sea or at anchor, are legal and common in shipping. Still, they are a favoured way to dodge sanctions, since they can let ships skip port inspections and hide where a cargo came from. The EU's latest sanctions package, adopted in July, for the first time targets vessels that refuel and service shadow fleet tankers at sea.

South Korea does not appear to have broken its own laws, which rely mainly on export controls for goods such as machinery, electronics and vehicles. Its foreign ministry did not address the shipments or the tankers directly. It said the government backs international efforts to end the war and is "strictly enforcing" its export controls on Russia. Seoul has sent Ukraine aid and non-lethal military supplies but has ruled out sending weapons directly.