A United Nations maritime tribunal has ruled that Russia must immediately release three Ukrainian naval vessels captured in November, and the 24 sailors it detained.
The International Tribunal for the Law of the Sea issued its order at its Hamburg headquarters following a hearing earlier this month. Russian representatives did not attend.
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 but which gives Russia a much bigger shoreline along the Black Sea. The Kerch Strait separates Crimea from mainland Russia.
Russia, which did submit a memorandum to the tribunal, had argued that the rights Ukraine claims in the case did not apply because they were covered by an exception for military activity.
Kiev's lawyers contest this claim, saying Russia itself has previously described the arrest as a law enforcement operation. The tribunal sided with Ukraine's argument on that point.
However, tribunal president Jin-hyun Paik said "the tribunal considers it appropriate to order both parties to refrain from taking any action which might aggravate or extend the dispute".
He said Russia must return the ships to Ukrainian custody and allow the service personnel to go home.
But the tribunal "does not consider it necessary to require (Russia) to suspend criminal proceedings against the 24 detained Ukrainian servicemen and refrain from initiating new proceedings", he added. Kiev had called for legal proceedings to be ended.
The tribunal's decisions are legally binding, but it has no power to enforce them. It called for both sides to report back on their compliance by June 25.
Paik said Russia "was given ample opportunity to present its observations" in the case. The tribunal made its decision by a 19-1 vote, with a Russian judge dissenting.
Olena Zerkal, Ukraine's deputy foreign minister for European integration, said on Facebook: "We expect that Russia will quickly and fully fulfil the tribunal's order."
In a statement, the Russian foreign ministry did not address the details of the order but underlined its argument that the UN Convention on the Law of the Sea's dispute resolution procedures cannot be applied to this dispute.
In subsequent arbitration proceedings at the tribunal, "we intend to consistently defend our position, including the lack of jurisdiction", it said.
Australian Associated Press