Ukraine says a Russian coast guard vessel has rammed one of its navy tugboats off the Moscow-annexed Crimean Peninsula in "openly aggressive actions," resulting in damage to the ship.
The Ukrainian Navy said the incident took place on November 25 as three of its ships including two small warships were transiting from the Black Sea port of Odesa to Mariupol in the Sea of Azov approached the Kerch Strait between Crimea and Russia.
Russia's Federal Security Service (FSB), which oversees the country's border guard service, accused Kyiv of illegally entering its waters and deliberately provoking a conflict.
The Sea of Azov has been an area of heightened tensions between Moscow and Kyiv since Russia seized Ukraine's Crimea region in March 2014 and shortly thereafter began supporting separatists in the eastern Ukrainian regions of Donetsk and Luhansk in a conflict that has killed more than 10,300 people.
A 2003 treaty between Russia and Ukraine designates the Kerch Strait and Sea of Azov as shared territorial waters.
However, Moscow has been asserting greater control over the passage in the past several years, particularly since it opened a 19-kilometer-long bridge linking Crimea with Russian territories across the Kerch Strait in May.
Both sides have recently increased their military presence in the Azov region, with Kyiv accusing Moscow of harassing ships heading for Ukrainian ports such as Mariupol.
A Russian border guard ship, the Don, "rammed into our tugboat," Ukraine’s navy said in a statement, adding that this caused damage to its engine, outer shell, and guardrail.
Russia's ships "carried out openly aggressive actions against Ukrainian naval ships," it also said, adding that its ships were continuing on their way "despite Russia's counteraction."
Meanwhile, the FSB said in a statement that the Ukrainian ships "illegally entered a temporarily closed area of Russian territorial waters.
It said that Ukraine’s ships were carrying out "provocative actions" and that their aim was to “create a conflict situation in this region."
It did not mention ramming the Ukrainian tugboat.
Ukraine said it gave Russian authorities advance warning of its ships' route.
The FSB later said that two Ukrainian gunboats left Berdyansk and were sailing toward the Kerch Strait, according to the Interfax news agency.
The security service also warned Kyiv against "reckless decisions" and said it was taking " all necessary measures to curb this provocation," Interfax reported.
Meanwhile, Russia's state-run television Rossia 24 TV channel said that Russia blocked traffic through the Kerch Strait by placing a cargo ship beneath the Kerch bridge. It also said that Russian military helicopters were circling overhead.