Ukraine has accused Russia of ramming one of its boats off the coast of the annexed Crimea, amid rising tensions.
The “openly aggressive actions” occurred as three of its vessels sailed from the Black Sea port of Odessa to Mariupol in the Sea of Azov, Kiev said.
Ukraine’s Yana Kapu tug suffered damage to its engine, guard rail and outer shell, the Ukrainian navy said.
But Russia said the ships were in its waters, and accused the Ukrainians of “provocative actions”.
Russia has now blocked access to the Sea of Azov with a tanker under the bridge that spans the Kerch Strait.
Tensions have risen in the seas off the Crimean peninsula – annexed by Russia in 2014 – in recent months.
What are the latest developments?
The Ukrainian navy said in a statement that its boats – two gunboats – were now being monitored by two Russian helicopters scrambled by Moscow.
The navy said the boats were waiting to cross into the Sea of Azov through the Kerch Strait, which Russia had shut off.
Russia said the traffic was suspended for security reasons.
Meanwhile, a Russian FSB security service official was quoted by Ria Novosti news agency as saying that Ukraine had sent naval reinforcements to the area.
The Sea of Azov lies east of the Crimean peninsula, and south of Ukraine’s regions partially seized by pro-Russian separatists.
In 2003, Ukraine and Russia signed a treaty on the Sea of Azov, which defined it as internal waters of the two countries. The treaty guaranteed free navigation to all Ukrainian and Russian vessels.
Russia said the Ukrainian vessels – two small warships named as the Berdyansk and the Nikopol, and the tug – were trying “to create a conflict situation in this region” after they left Odessa.
The ships “illegally entered a temporarily closed area of Russian territorial waters”, the FSB reportedly said in a statement.
But Ukraine said it had informed the Russians of its plan to move its ships through the sea to Mariupol.
The navy statement said the Russian actions marked a breach of the UN Convention on the Law of the Sea.
Russia has recently increased its military presence and inspections, with some calling its actions an “economic blockade” of Ukraine’s Azov ports in the east of the country.
More than 10,000 people have died in the eastern Donetsk and Luhansk region since separatists moved against the Ukrainian state in April 2014.
Ukraine and the West accuse Russia of sending its troops to the region and arming the separatists.
Moscow denies this, but admits that Russian “volunteers” are helping the rebels.