Nathan Hodge, CNN - The Ukrainian military said Sunday that Russian boats had opened fire on and seized three of its ships near Crimea, escalating a standoff over the Kerch Strait, a waterway that links the Azov Sea with the Black Sea.

In a statement, the Ukrainian Ministry of Defense said the small gunboats Berdyansk and Nikopol and the tugboat Yana Kapu were attacked.

"Ships seized by Russian special forces," the statement read

Six of its sailors were injured in the incident, Ukraine's navy said, adding the tugboat and the Berdyansk were disabled by Russian forces and were being towed away by the Russian navy. The undamaged Nikopol also is under Russian control, Ukraine said.

Earlier, Russian authorities closed off the Kerch Strait amid the confrontation with the Ukrainian naval vessels, Russian state news agencies and the Ukrainian military said.

Ukraine and Russia offered conflicting accounts of the incident at sea.

Russian state news agency RIA-Novosti, citing Russia's Federal Security Service, or FSB, said the three vessels of the Ukrainian navy had reversed course away from Kerch Strait.

The FSB's Border Service in Crimea reported that three Ukrainian warships had illegally entered Russia's territorial waters, and were carrying out dangerous maneuvers, according to Russian state news agency TASS.

According to RIA-Novosti, the FSB blamed Kiev for provocations in the waters of the Black Sea.

The Ukrainian navy said Russian border patrol vessels "carried out openly aggressive action" against the Ukrainian ships.

The office of Ukrainian President Petro Poroshenko described the Russian action against the Ukrainian navy ships as "an act of aggression aimed at deliberately escalating the situation in the waters of the Sea of Azov and the Kerch Strait," and called for international condemnation of Moscow and the imposition of new sanctions.

Poroshenko convened a military cabinet emergency meeting.

In a statement late Sunday, Secretary of the National Security and Defense Council of Ukraine Oleksandr Turchynov said Ukraine's National Security and Defense Council will discuss the introduction of martial law after a naval incident between Russian and Ukrainian vessels around the Kerch Strait off the coast of Crimea.

The Azov Sea is an important economic lifeline for Ukraine, as it links the port city of Mariupol with the Black Sea. Both Ukraine and Russia share the Azov Sea: According to TASS, a 2003 treaty confirms the Azov Sea and the Kerch Strait as domestic waters of Russia and Ukraine.

In a statement to CNN, NATO spokesperson Oana Lungescu said NATO "fully supports Ukraine's sovereignty and its territorial integrity" and is calling on Russia "to ensure unhindered access to Ukrainian ports in the Azov Sea." The European Union echoed the latter sentiment.

"NATO is closely monitoring developments in the Azov Sea and the Kerch Strait, and we are in contact with the Ukrainian authorities. We call for restraint and de-escalation," the statement said.

Russia and Ukraine have been locked in conflict since the annexation of the Black Sea peninsula of Crimea by Russia in 2014, and the incident at sea raises new tensions between the two countries.

Russia has backed separatists in Ukraine's eastern Donbas region, and earlier this year, Russia solidified its hold over Crimea, opening a road linking Russia's Krasnodar region with the Crimean Peninsula.

The international community has largely not recognized Russia's annexation of the Black Sea peninsula of Crimea from Ukraine. According to TASS, a 2003 treaty confirms the Azov Sea and the Kerch Strait as domestic waters of Russia and Ukraine.

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