Ukraine on Thursday seized a Russian tanker it said was used in a naval confrontation last November amid sensitive prisoner swap talks between the two countries, at loggerheads since 2014.

Kiev's SBU security service said it stopped the tanker, the Neyma, as it entered the port of Izmail in the southern Odessa region. Investigators seized documents onboard and questioned crew members, an SBU statement said.

Moscow responded by threatening Kiev with unspecified ‘consequences’.

The move comes two weeks after Ukraine's new leader Volodymyr Zelensky and Russia's Vladimir Putin discussed a possible prisoner swap during their first phone call.

Russia has held 24 Ukrainian sailors since it seized three Ukrainian vessels in the November confrontation -- the most dangerous clash between the two countries in years.

The ex-Soviet neighbours have been stuck in hostilities since 2014 when Moscow annexed Crimea and supported an insurgency in eastern Ukraine. Some 13,000 people have been killed in the conflict.

Following the incident Thursday, a representative of the Russian foreign ministry told AFP that Moscow was ‘looking into what happened to take appropriate measures.’

‘If Russians are taken hostage, this would be interpreted as the crudest violation of international law and consequences will not be long in coming,’ said the official.

- Name changed -

According to the SBU, the tanker participated in an operation by Russia to seize three Ukrainian vessels off Crimea last November. The Neyma had changed its name painted on the hull to ‘conceal its involvement in the act of aggression,’ it said.

The SBU released a video showing Ukrainian officials on the tanker, which had a Russian flag on it.

Russian officials said Ukraine eventually released crew members without charge but kept the Neyma.

‘The crew is headed home... the vessel is in Izmail,’ said Russian embassy spokesman Denis Golenko to Interfax news agency.

There are ten Russian crew members ‘being put on a bus to Moldova, they will fly to Moscow from there,’ the agency quoted the office of Russian ombudsman Tatyana Moskalkova as saying.

Ukrainian analyst Sergiy Solodky told AFP that the move likely took Russia by surprise, saying Moscow had hoped for Kiev to have a softer position on Russia after Zelensky took office in May.

‘This will provoke an angry reaction from the Russian leadership,’ he said.

Last week Russia prolonged the detention of the 24 Ukrainian sailors, who remain imprisoned in Moscow.

The International Tribunal for the Law of the Sea, based in the German port city of Hamburg, has urged Russia to release the sailors and return them to Ukraine.

Moscow accuses the sailors of violating its maritime borders.

Related Story