Ukraine has accused Russia of using a coast guard vessel to ram one of its navy tugboats off the coast of the annexed Crimea.
It was travelling on Sunday 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 news agencies the Ukrainian ships held their course and violated Russian territorial waters, which were temporarily closed.
The FSB accused Ukraine's 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.
Russia has blocked passage through the Kerch Strait, a narrow body of water nestled between Crimea and the Russian mainland.
The move came after three Ukrainian navy ships made what the Russian coast guard has called an unauthorised crossing through "Russian territorial waters".
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.
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.
