Kyiv says it has detained a Russian tanker it alleges was involved in an incident in the Kerch Strait that saw three Ukrainian vessels seized.
Their crews were also captured after the seizure in November 2018 and remain in custody.
Last year's incident happened in waters separating Russia and Crimea, which Moscow annexed from Ukraine in 2014.
Ukraine's secret service (SBU) said in a statement on Thursday the Russian tanker — formerly named the Neyma but now called Nika Spirit — alleged to have been involved in the November 2018 incident had been detained in the Ukrainian port of Izmail.
The vessel remains in Ukrainian custody but its crew have been freed and were on their way home, according to a spokesman for the Russian Embassy in Kyiv.
"An investigation has established that Russian owners changed the name of 'Neyma' to 'Nika Spirit' in order to hide its involvement in the illegal acts of aggression that took place on November 25, 2018," said SBU.
“The above-named vessel is considered to be a piece of material evidence, (and) a petition to a court for its arrest is being prepared."
Senior Russian lawmaker Vladimir Dzhabarov said Ukraine's move was "absolutely illegal", reported RIA news agency.
Ukraine’s ombudsperson, Lyudmyla Denisova said negotiations on the release of the Ukrainian sailors from the three vessels allegedly seized by the Russian navy off Crimea’s coast had intensified after Russian and Ukrainian leaders spoke by phone earlier this month.
“Negotiations on the release of Ukrainian sailors have moved after a telephone conversation between the president of Ukraine and the president of Russian Federation,” Denisova told a news conference.