The International Tribunal for the Law of the Sea on Saturday declared that the Ukrainian sailors detained by Russian authorities must be released with immediate effect.
The 24 sailors and three naval vessels were seized by Russia off the Crimean peninsula in November 2018.
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Judge Jin-Hyun Paik, of the International Maritime Court in the German city of Hamburg, said, "The Russian Federation must proceed immediately to release the Ukrainian soldiers and allow them to return to Ukraine."
The tribunal's decision was a 19-1 vote with only the Russian judge supporting Moscow.
Russia did not attend a hearing on the case earlier this month or the session to hear the judge's ruling and had argued earlier that the court did not have jurisdiction to rule on the incident. Moscow had accused the sailors of violating its maritime borders.
A treaty between Russia and Ukraine gives both countries the right to use the Sea of Azov, which is linked to the Black Sea by the Kerch Strait, where the sailors were taken into Russian custody.
Newly elected Ukrainian President Volodymyr Zelenskiy said Russia could send a signal of "real readiness to stop the conflict with Ukraine" by complying with the tribunal's order.
Russia annexed Crimea in 2014 in a move most of the world, including Germany, has deemed illegal. Since then fighting between pro-Russian separatists and Kiev government forces in the east of Ukraine has seen 13,000 lose their lives, according to United Nations figures.
Ukraine's former President Petro Poroshenko insisted the sailors were prisoners of war and described their incarceration as "blatant proof that Russia continues to show cynical disrespect for human rights."
jsi/sms (AFP, AP, Reuters)