The United Nations Security Council will hold an emergency meeting on Monday after Russia seized three Ukrainian navy ships off the coast of Crimea, the US ambassador to the UN, Nikki Haley, wrote on Twitter.
Russia's Federal Security Service (FSB), which oversees the country's border guard service, said it was forced to open fire on the ships on Sunday after they illegally entered Russian territorial waters.
"There is irrefutable evidence that Kyiv prepared and orchestrated provocations," it said. "These materials will soon be made public."
Three Ukrainian sailors wounded in the clashes were in a stable condition and receiving medical care, it added.
Earlier on Sunday, Russia blocked the Kerch Strait, a narrow passage between Crimea and the Russian mainland, accusing the same three Ukrainian vessels of entering its territorial waters without permission.
The two Ukrainian navy artillery boats and a tugboat were transiting from Odessa on the Black Sea to Mariupol in the Sea of Azov via the Kerch Strait.
The Ukrainian navy said a Russian coast guard vessel responded to their presence by ramming the tugboat, resulting in damage to the ship's engines and hull.
Russia has since reopened the Kerch Strait for shipping, the Russian news agency RIA said on Monday, citing a port official.
EU, Germany, NATO call for restraint
Ukraine said Russia's actions broke international law and vowed a "diplomatic legal response." The FSB, it insisted, had received advance notice about the ships' movements.
President Petro Poroshenko said on Monday that he would propose parliament declare martial law. This would restrict civil liberties and give state institutions greater power.
The European Union and NATO separately called for restraint on both sides and for Russia to restore freedom of passage via the Kerch Strait.
German Foreign Minister Heiko Maas has called the developments in the Sea of Azov "worrying" and said that it was "not acceptable to have a Russian blockade there." But he said it was "a good sign" that Russia had decided to reopen the Kerch Strait.
Although a 2003 treaty designates the tow areas as shared territorial waters, Russia has been asserting greater control over the passage since 2015.
The latest dispute has raised concerns about a possible escalation of a wider conflict between the two neighbors.
Russia annexed the Crimean Peninsula in 2014 to widespread international condemnation. Moscow's military intervention in the peninsula and subsequent referendum inflamed separatist sentiment in eastern Ukraine, effectively triggering a rebel insurgency by pro-Russia forces.
dv, amp/ng (dpa, Reuters)