A South Korean ship has been detained for six months on suspicion of violating UN sanctions against the nuclear-armed North.
Pyongyang is sanctioned over its weapons programme under multiple UN Security Council resolutions, including curbs on oil and fuel shipments.
But the North has been seeking to get around the measures. A recent UN report said that Pyongyang was securing fuel deliveries through ship-to-ship transfers in international waters.
Seoul's foreign ministry said a South Korean ship and three others had been detained on
suspicion of undermining the economic measures against the North.
The ship - held in the southern port of Busan since last October - is suspected of transferring fuel to a North Korean tanker. If it is proved the South Korean vessel is likely to be blacklisted by the UN.
South Korean President Moon Jae-in has sought to engage the North to bring it to the negotiating table, dangling economic carrots before it in the form of inter-Korean projects, many of which would violate sanctions rules if implemented.