A U.N. maritime tribunal has ruled that Russia must release three Ukrainian naval vessels captured by Russia in November and release 24 detained sailors.

The Hamburg-based International Tribunal for the Law of the Sea delivered its ruling Saturday on the case Ukraine brought against Russia.

The confrontation in the Kerch Strait, which links the Sea of Azov with the Black Sea, marked a flashpoint in the simmering conflict over Russia’s 2014 annexation of the Crimean Peninsula. Russia seized Crimea in a move that Ukraine and most of the world view as illegal.

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Tribunal president Jin-Hyun Paik said that judges decided Russia must “immediately” return the three ships to Ukraine’s custody, and release the sailors and allow them to return to Ukraine.