Ukraine’s defence ministry has warned Russia it will soon send navy ships through the Kerch Strait where Russia fired on and seized three Ukrainian vessels two weeks ago.

The announcement sets up another possible flashpoint in the long-simmering conflict between Russia and Ukraine that erupted in 2014 with Russia’s annexation of the Crimean peninsula burst into the open on November 25 when Russian border guards opened fire on three Ukrainian navy vessels that were moving through the Kerch Strait near Crimea and seized the ships and their crews.

Ukraine responded by introducing martial law for 30 days, a measure Kiev did not take even after Crimea’s annexation and amid large-scale fighting between Ukrainian forces and Russia-backed separatists in 2014-2015.

Ukraine Russia
A woman with her child walks in the harbour of Mariupol, in the Sea of Azov, eastern Ukraine (Evngeniy Maloletka/AP)

As part of martial law, Ukraine has beefed up its forces on the border with Russia and called up reservists for training.

Ukrainian defence minister Stepan Poltorak told reporters on Friday that his country intends to send naval ships through the Kerch Strait soon, saying that “otherwise Russia will fully occupy the Sea of Azov”.

Ukraine has ports both on the Black Sea and on the Sea of Azov which are linked by the Kerch Strait that separates Crimea from Russia.

More than 140 ships were stuck on both sides of the strait on Friday due to excessive checks and delays on the Russian side, the Ukrainian Border Guard Service said, accusing Russia of creating a bottleneck for Ukrainian ships and preventing them from passing.

Ukraine Russia
Harbour cranes are seen in Mariupol trade port in Ukraine (Efrem Lukatsky/AP)

Speaking in Rome at a ministerial meeting of the Organisation for Security and Cooperation in Europe, Russian foreign minister Sergey Lavrov brushed off suggestions Russia could release or exchange the 24 Ukrainian seamen captured in last month’s maritime confrontation.

Mr Lavrov said it was too early to speak about any offers of a prisoner swap until Russia’s investigation of the incident is completed and the servicemen are put on trial for violating the Russian border.

Mr Lavrov also reacted to a suggestion by German foreign minister Heiko Maas to expand the Osce monitoring mission to the Sea of Azov, saying that Russia “has no need” for any intermediaries or monitors in the area.

Switzerland
Russian foreign minister Sergey Lavrov (Martial Trezzini/AP)

Russian ombudswoman Tatyana Moskalkova told Russian news agencies that she visited three Ukrainian seamen who were injured in the incident and are being held in a Moscow prison hospital.

Ms Moskalkova said she spoke to the men and “got the impression” that the crew “were not aware that they were committing a crime”.

Meanwhile, a court in Crimea on Friday jailed for five days lawyer Emil Kurbedinov, who represents the commander of one of the Ukrainian ships.

The court found Kurbedinov guilty of spreading extremist material in a social media post he made in 2013.