Ukraine mobilized all of its naval personnel and ships after Russia seized three of its vessels off the coast of Crimea, which it annexed in 2014.
Ukraine called an emergency meeting of what it called its war cabinet on Sunday after it accused Russia of having fired on three of its vessels in the Black Sea, injuring at least six sailors.
Russia's Federal Security Service, or FSB, confirmed that it seized what it called three Ukrainian naval "warships," saying they had trespassed into Russian territorial waters. It said "weapons were used to force the Ukrainian warships to stop," TASS, the official Russian news agency, reported Sunday night.
The Ukrainian Foreign Affairs Ministry said it had mobilized all naval personnel and had sent all of its ships to sea after what it described as two gunboats and a tugboat coming under attack off the coast of Crimea, which Russia annexed in 2014.
Oleksii Makeiev, political director of Ukraine's Foreign Affairs Ministry, called the incident an act of "warmongering" that "undermines security of the whole region." The country's National Security and Defense Council said in a statement that the war cabinet would meet Sunday night to consider imposing martial law.
Russia's seizure of the naval vessels would represent an escalation of tensions that flared a few hours earlier, when it blocked all traffic under the Crimean Bridge into the Kerch Strait — the only passage into the Azov Sea — because of what it said was an approach of Ukrainian vessels, TASS said.
The FSB said Ukraine failed to notify Russia that its ships were planning to use the strait, a contention that Ukraine denied. While both Ukraine and Russia have rights to the sea under a 2003 treaty, Russia controls both sides of the strait.
"The vessels are carrying out dangerous maneuvers and are disobeying the Russian authorities' demands," the FSB's Border Service said in a statement. "The Border Service is taking all steps to ensure security of navigation and regulation of maritime traffic in the waters of the Black Sea and the Sea of Azov and the Kerch Strait."
The government of Ukrainian President Petro Poroshenko called the Russian actions "an act of aggression aimed at deliberately escalating the situation in the waters of the Sea of Azov and the Kerch Strait."
NATO is closely monitoring the developments and called for restraint, said Oana Lungescu, a NATO spokeswoman.
"NATO fully supports Ukraine's sovereignty and its territorial integrity, including its navigational rights in its territorial waters," she said in a statement. "We call on Russia to ensure unhindered access to Ukrainian ports in the Azov Sea, in accordance with international law."
Similarly, the European Union said that it expected Russia "to restore freedom of passage at the Kerch strait" and that it urged "all to act with utmost restraint to de-escalate the situation immediately."
"The EU does not and will not recognize the illegal annexation of the Crimean peninsula by Russia" in 2014, the union said in a statement.
The United States and Russia clashed in July after the White House rejected Russian President Vladimir Putin's proposal to organize an indeendence referendum in separatist-leaning eastern Ukraine.