Russia has opened fire on Ukrainian ships and captured three vessels in a major escalation of tensions off the coast of Crimea.
Three sailors have been wounded after the Ukrainian navy said two artillery boats were hit in the strikes.
Ukraine’s President Petro Poroshenko has called an emergency session of his war cabinet in response to the incident in the Black Sea.
Russia’s Federal Security Service (FSB) said it had used weapons after the Ukrainian ships ignored demands to stop and impounded three vessels which had illegally crossed the border.
The three injured sailors are receiving medical treatment and their lives are not in danger, the FSB said.
But Ukraine’s ambassador to the UK said Russian special forces had captured two armoured artillery boats and a tugboat in an “act of aggression”.
“Today’s dangerous events in the Azov Sea testify that a new front of Russian aggression is open,” Ukrainian foreign ministry spokeswoman Mariana Betsa said.
“Ukraine is calling now for an emergency meeting of the United Nations Security Council.”
In response, the FSB said it had “irrefutable evidence that Kiev prepared and orchestrated provocations… in the Black Sea”.
“These materials will soon be made public,” it added.
Earlier on Sunday, Ukraine accused a Russian coastguard vessel, named the Don, of ramming one of its tugboats, damaging its engine, hull and side railing.
Ukraine’s minister of internal affairs posted footage on Twitter purportedly showing the incident.
It allegedly took place as three Ukrainian navy boats – including two small warships – headed for the port of Mariupol in the Sea of Azov, an area of heightened tensions between the countries.
Russia – which claims the waters off Crimea after annexing the peninsula in 2014 – accused Ukraine of illegally entering the area and deliberately provoking a conflict.
It placed a huge cargo ship beneath the Russian-controlled Kerch Strait Bridge to block Ukrainian boats from access to the sea.
“Their goal is clear – to create a conflict situation in the region,” the FSB said in a statement.
The Ukrainian navy insisted Russia had been informed in advance about the planned journey.
“Russian coastguard vessels… carried out openly aggressive actions against Ukrainian navy ships,” it said.
The European Union called on Russia and Ukraine to “act with utmost restraint to de-escalate” the situation in the Black Sea.
It urged Russia to “restore freedom of passage” through the Kerch Strait after Moscow blockaded it.
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NATO also demanded Russia ensures “unhindered access to Ukrainian ports in the Azov Sea, in accordance with international law”.
A 2003 treaty designates the Kerch Strait and Sea of Azov as shared territorial waters but Russia has been asserting greater control over the area since 2015.
In September, the Ukrainian navy accused Russian border guards of “acts of provocation” against its ships taking the same route.
Ukraine has increased the number of navy ships and border guard patrols in the Sea of Azov, which is reached via the Kerch Strait between Crimea and Russia.
Kiev says its naval build-up is due to Russia stepping up controls in the area this year and carrying out lengthy checks on commercial shipping.
Mariupol is close to the region of eastern Ukraine controlled by Russian-backed separatists in a conflict that has caused at least 10,000 deaths since 2014.