An international tribunal is due to rule Saturday on a dispute over Russia's holding 24 Ukrainian sailors and three naval vessels seized off the Crimea peninsula late last year.
Judge Jin-Hyun Paik of the International Tribunal for the Law of the Sea in the German port city of Hamburg was scheduled to announce its decision at noon (1000 GMT).
The incident in November in the Kerch Strait was the most dangerous direct clash in years between Russia and its ex-Soviet neighbour, which have been locked in conflict for over five years.
Russia in March 2014 annexed Crimea, and since then fighting between pro-Russian separatists and Kiev government forces in Ukraine's east has claimed some 13,000 lives, according to UN figures.
Ukraine's former president Petro Poroshenko insisted the sailors were "prisoners of war" and described their detention as "blatant proof that Russia continues to show cynical disrespect for human rights".
Moscow has accused the sailors of violating its maritime borders, and lawyers have said they could face up to six years in prison.
Kiev on May 10 asked the international tribunal to order Russia to release the sailors and the Berdyansk, Nikopol and Yani Kapu vessels.
Ukrainian foreign ministry spokeswoman Kateryna Zelenko told AFP she hoped for judicial support to help "break the deadlock on the release of sailors and military vessels".
Russia has denied any wrongdoing and said it does not recognise the jurisdiction of the court, a body with 168 signatories created under the United Nations Convention on the Law of the Sea, over the case.
Bogdan Bezpalko, a member of the Kremlin's International Relations Committee, said recently on Sputnik Radio that "Russia has not violated international law; Russia has reacted to an aggression against it."
- Stalemate -
Earlier this month Ukraine's deputy foreign minister Olena Zerkal, addressing the tribunal, described the sailors' detention as "an additional illustration of Russia's continued disrespect for international law".
"Each additional day of detention, each interrogation, each court case aggravates the dispute between the parties," she said.
Russia has argued that the rights of the Ukrainian sailors have been strictly observed, including access to legal aid and medical attention.
French President Emmanuel Macron and German Chancellor Angela Merkel jointly phoned Russian President Vladimir Putin on Tuesday to encourage him to resume the dialogue with Ukraine after the election of the new president, Volodymyr Zelensky.
US Secretary of State Mike Pompeo recently called on Moscow to show Zelensky a willingness to move toward "breaking the stalemate".
Pompeo said Washington "would in particular welcome the release of the Ukrainian crewmen detained near the Kerch Strait last year".
But since Zelensky was sworn in on Monday, tensions have risen with Moscow.
The new Ukrainian leader has urged Washington to impose more sanctions against Russia -- prompting a swift warning from the Kremlin that such action would not help end the war in the east.