The Ukrainian navy has accused Russia of opening fire on some of its ships off the coast of the annexed Crimea.
Sunday's announcement was made after a day of rising tensions during which Moscow accused the Ukraine of illegally entering Russian waters.
Russia placed a huge cargo ship under a bridge in the Kerch Strait to block three Ukrainian navy vessels from entering the Sea of Azov.
Ukraine's navy claimed Russian fire damaged two small artillery boats and injured two crewmembers.
It added that the two ships, plus another, were then seized by Russian Special Forces.
Ukrainian president Petro Poroshenko has called an emergency session of his war cabinet to discuss the situation.
Russia is yet to comment on the allegations.