Russia and Ukraine accused each other of breaking international law on Sunday after Russian border guards tried and failed to stop two Ukrainian armoured artillery vessels circumnavigating Crimea en route to a Ukrainian port.
The incident in the Black Sea reflects rising tensions between Moscow and Kiev around the Sea of Azov which both countries have the right to use.
Russia and Ukraine, whose relations remain acutely strained after Moscow's 2014 annexation of Crimea and its backing for a pro-Russian insurgency in eastern Ukraine, have accused each other of harassing each other's shipping in the area.
On Sunday, the Ukrainian Navy said it had sent two small armoured artillery vessels and a tug boat from the port of Odessa to Mariupol, a journey that requires them to pass from the Black Sea via the Kerch Strait into the Sea of Azov.
It said in a statement that Russian border guard vessels had tried to stop the small flotilla by ramming the tug boat, causing minor damage to it in an incident it said showed Russia was behaving aggressively and illegally.
Russian news agencies reported that Russia's FSB security service had accused Ukraine of not informing it in advance of the journey, something Kiev denied, and had said the Ukrainian ships had been manoeuvring dangerously with the aim of stirring up tensions.
The Ukrainian navy said the flotilla was continuing its journey regardless.
