Russia has fired on and seized three Ukrainian naval vessels off the Crimean Peninsula in a major escalation of tensions between the two countries.

Two gunboats and a tug were captured by Russian forces. A number of Ukrainian crew members were reportedly injured.

Each side blames the other, and the Ukraine government said it could declare martial law.

Russian navy ships intercepted the Ukrainian vessels after accusing them of illegally entering Russia’s waters.

Ukrainian President Petro Poroshenko called an urgent meeting of his “war cabinet” over the incident, his spokesperson said.

The incident began when Ukraine’s Berdyansk and Nikopol gunboats, and the Yana Kapa tug, tried to sail from the Black Sea port of Odessa to Mariupol in the Sea of Azov.

Ukraine says the Russians tried to intercept the ships, ramming the tug. The vessels continued towards the Kerch Strait, but were prevented by the tanker.

A tanker under the bridge shut all navigation from and into the Sea of Azov.

Russia scrambled two fighter jets and two helicopters to the area. It accused the ships of illegally entering its waters and said the traffic had been suspended for security reasons.

The Ukrainian navy later said the boats had been hit and disabled as they tried to leave the area. It said six crew members had been injured.

Ukraine said it had informed the Russians of its plan to move its ships through the sea to Mariupol.

Tension between Russia and Ukraine has been building for months off Crimea.

Under a 2003 treaty between Moscow and Kiev, the Kerch Strait and the Sea of Azov are shared territorial waters.

But recently there, Russia began inspecting all vessels sailing to or from Ukrainian ports.

The European Union called on Russia to “restore freedom of passage at the Kerch Strait” and urged “all to act with utmost restraint.”

NATO said it “fully supports Ukraine’s sovereignty and its territorial integrity, including its navigational rights in its territorial waters.” It said Russia should “ensure unhindered access to Ukrainian ports in the Azov Sea.

Ukraine has requested an emergency meeting of the U.N. Security Council and has called for international action against Moscow.

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