The Russian military opened fire on three Ukrainian vessels in the Black Sea on Sunday, and seized two naval artillery boats, in the latest aggression by Russia President Vladimir Putin against NATO interests.
The waters where the vessels were attacked are, under a 2003 treaty between Moscow and Kiev, considered "shared" territory.
The incident began early in the day, when Ukraine's navy claimed that the Russian navy had "rammed" the tugboat that was escorting the two artillery ships. Russia blocked the strait in retaliation for the accusation, and tensions escalated, eventually resulting in the seizure.
The disputed area, the Kerch Strait, is spanned by a bridge connecting Russia and Ukraine. That area is the only way into the Sea of Azov from Odessa on the Black Sea. It is under that bridge that the Russians blocked passage, using ships.
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Ukraine's Foreign Ministry said in a statement that Russia's actions "have transgressed all boundaries and become aggressive ... [and] demand clear reaction of international community."
"We consider such aggressive actions as a violation of the norms of the UN Charter and the United Nations Convention on the Law of the Sea," the statement went on. "Ukraine will take all appropriate measures to ensure diplomatic and legal response."
European Union foreign affairs spokeswoman Maja Kocijanic called on both Russia and Ukraine to "act with utmost restraint to de-escalate" the situation.
Ukrainian President Petro Poroshenko called an emergency meeting of his cabinet, and has spoken with EU and NATO leaders over the increasingly dangerous situation and what he referred to as "criminal acts" on the part of Russia.
The EU issued a statement. "We expect Russia to restore freedom of passage at the Kerch strait and urge all to act with utmost restraint to de-escalate the situation immediately," it reads. It points out that the bridge in question was built without the consent of the Ukraine, and accuses Russia of "disregarding" basic rules of internetional cooperation.
The statement closes by saying ""he EU does not and will not recognise the illegal annexation of the Crimean peninsula by Russia."
Two Ukrainian sailors were injured during the Russian attack.