The Ukrainian Security Service said on Thursday it had seized a Russian tanker in a Black Sea port, in a move that could undermine an anticipated prisoner swap between the two countries. The service said it detained the Russian tanker in a Ukrainian port on Wednesday. Authorities said the tanker was involved in blocking Ukrainian vessels from sailing through the Kerch Strait in November. Russia claimed the vessels breached its territorial waters. The 10 crew members of the Russian tanker were allowed to disembark and leave Ukraine because they were not involved in November's incident, Russian human rights ombudswoman Tatyana Moskalkova said. Russian ships fired on and seized the Ukrainian vessels on November 25 in the Kerch Strait, between the Black Sea and the Sea of Azov. Ukraine has insisted the vessels were in international waters when Russia intercepted them. Officials in both countries said this month that they were preparing for a major prisoner exchange that would include the 24 Ukrainian crew members detained in November. UN deputy spokesman Farhan Haq said the world body could not verify the circumstances surrounding the incident but repeated that both parties should refrain from making matters worse, through words or action. Ukrainian President Volodymyr Zelenskiy's office did not have immediate comment on the incident. Russian officials protested against the tanker's seizure, saying it would dampen the chances of the prisoner swap. "This is very bad news," Russian Deputy Foreign Minister Grigory Karasin told the state-owned RIA Novosti news agency on Thursday. "Someone in Kiev wants a tough throwback to their own provocation on November 25." The Ukrainian Security Service is still led by an ally of Ukraine's former president, Petro Poroshenko, Mr Zelenskiy called Russian President Vladimir Putin this month for their first conversation since the Ukrainian took office in May. Mr Zelenskiy, who was a popular comedian before he won an election in April, made securing the release of the Ukrainian sailors one of his first priorities.