Ukraine's president has convened an emergency military meeting after two of its ships were allegedly hit by fire from Russia near the Crimea.
Petro Poroshenko is to gather the country's top military brass after the two navy vessels were damaged and two crew members were injured on Sunday evening.
Ukraine said that the ships were then seized by Russia, as tensions around the annexed region continue to rise.
The incident came after Russia supposedly blocked passage through the Kerch Strait, a narrow body of water nestled between Crimea and the Russian mainland.
This followed what the Russian coast guard called an unauthorised crossing through "Russian territorial waters" by three Ukrainian ships.
The European Union called on both sides to "act with utmost restraint to de-escalate" the situation in the Black Sea.
It follows months of growing tensions between Ukraine and Russia, which annexed Crimea from Ukraine in 2014 and has steadily worked to increase its zone of control around the peninsula.
The Kerch Strait is the only passage into the Sea of Azov beyond it. The strait is crossed by the recently completed Kerch Bridge, connecting Crimea to Russia.
Transit under the bridge has been blocked by a tanker ship and dozens of cargo ships awaiting passage are stuck.
The incident began earlier on Sunday when the Ukrainian navy claimed a Russian coast guard vessel rammed a Ukrainian navy tugboat.
It was travelling with two Ukrainian navy artillery boats from Odessa on the Black Sea to Mariupol in the Sea of Azov, via the Kerch Strait.
“Russian coast guard vessels … carried out openly aggressive actions against Ukrainian navy ships” during the transit, the Ukrainian navy statement said.
It said a Russian coast guard ship damaged the tugboat’s engine, hull, side railing and a lifeboat.
The statement added Russia was informed in advance about the planned transit.
Russia’s Federal Security Service, known as the FSB, told Russian news agencies the Ukrainian ships held their course and violated Russian territorial waters, which were temporarily closed.
The FSB accused the Ukrainian navy of staging a provocation against Russia.
“Their goal is clear,” an FSB statement said, “to create a conflict situation in the region."
The statement did not mention ramming a Ukrainian tugboat.
Though a 2003 treaty designates the Kerch Strait and Sea of Azov as shared territorial waters, Russia has been asserting greater control over the passage since 2015.
The Ukrainian Foreign Ministry in a statement said Russia’s actions were a violation of the UN Charter.