Two armored artillery boats of the Ukrainian Navy were reportedly floated off the shores of the town of Berdiansk on Sept. 16.
Ukrainian President Petro Poroshenko earlier warned that Russia's purpose is to occupy the Sea of Azov the same way it did Crimea.
"This is a brutal violation of international law, and we cannot tolerate it. We are strengthening our military footprint there and launching a case against Russia in the international Permanent Court of Arbitration. We have absolutely clear legal status in the Sea of Azov. Russia has no right to attack or stop our vessels, which carry goods and passengers from two important Ukrainian ports, Mariupol and Berdiansk. If Russia does not stop, we have only one instrument, which is sanctions," Poroshenko told The Washington Post in an interview published on Sept.13.