Ukraine has claimed two navy crew members were wounded after Russia fired at two of its vessels.
The Ukrainian navy said two of its vessels were struck on Sunday and Russian coast guard crews boarded them and a tugboat and seized both of them.
It said Russian ships fired on and seized two of its artillery ships following an incident near Crimea, which Moscow annexed from Kiev in 2014. A tugboat was also seized.
The Russian Federal Security Service claimed it has evidence Ukraine is responsible for clashes between ships in the Black Sea.
The agency, known as the FSB, said in a statement on Sunday night “there is irrefutable evidence that Kiev prepared and orchestrated provocations ... in the Black Sea”.
It added: “These materials will soon be made public.”
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Russian Foreign Ministry spokeswoman Maria Zakharova said on Facebook the incident was characteristic of Ukrainian behaviour - provoke, pressure and blame for aggression.
There have been 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.
Earlier on Sunday, Russia and Ukraine traded accusations over another incident involving the same three vessels, prompting Moscow to block passage through the Kerch Strait.
The Ukrainian vessels apparently wanted to travel through the strait to other ports in Ukraine.
Russia’s coast guard said the three Ukrainian vessels made an unauthorised crossing through its territorial waters, while Ukraine alleged one of its boats was rammed by a Russian vessel.
“Russian coast guard vessels ... carried out openly aggressive actions against Ukrainian navy ships,” the Ukrainian navy statement said.
It said a Russian coast guard ship damaged the tugboat’s engine, hull, side railing and a lifeboat.
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The statement added Russia had been informed in advance about the planned journey.
Russia then blocked off the strait.
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.
Russia has not given any indication of how long it will keep the strait blocked off but a long-term closure to civilian traffic would amount to an economic blockade of Ukrainian cities on the Azov coast.
Ukrainian cities on the Sea of Azov include strategically vital centres such as Mariupol - the closest government-controlled city to Donetsk and Luhansk, the breakaway regions of eastern Ukraine controlled by Russia-backed separatists.
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The FSB told Russian news agencies after the first incident that the Ukrainian ships held their course and violated Russian territorial waters.
it 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 said in an earlier statement Russia’s actions were a violation of the UN Charter and international law, and pledged to “promptly inform our partners about Russia’s aggressive actions.”
“Such actions pose a threat to the security of all states in the Black Sea region and therefore require a clear response from the international community,” it said.