Moscow (CNN)The Ukrainian military said Sunday that Russian boats opened fire on and seized three of its ships near Crimea, escalating a standoff over the Kerch Strait, a key waterway that holds strategic importance for both countries.

The incident marks a major escalation in tensions between the two former Soviet Republics. On Monday, Ukrainian lawmakers will vote on whether to declare martial law, following a proposal by the country's President Petro Poroshenko. An emergency United Nations Security Council meeting has been called for Monday to discuss the issue.In a statement, the Ukrainian Ministry of Defense said the small gunboats Berdyansk and Nikopol and the tugboat Yana Kapu were attacked.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.The Ukrainian Navy says its ships were planning to enter the Kerch Strait to access the city of Mariupol before they were fired upon by Russian forces. The strait connects the Azov Sea with the Black Sea and runs between the Crimean Peninsula and Russia. It's a shallow, narrow stretch of water just two to three miles (3.2 to 4.8 kilometers) wide at one point near the Chuska landspit.The strait is an important economic lifeline for Ukraine, as it allows ships leaving the port city of Mariupol to access the Black Sea.

What happened

Ukraine and Russia offered conflicting accounts of the incident, each accusing the other side of violating the laws of the sea. A 2003 agreement confirms the Azov Sea and the Kerch Strait as domestic waters of Russia and Ukraine.Russia's Federal Security Service (FSB) 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.TASS reported the three Ukrainian vessels were detained by Russian forces and that "weapons were used to force them to stop.""They did not respond to legitimate demands by the ships and boats of Russia's FSB Border Guard Service escorting them to stop immediately and performed dangerous maneuvers," the FSB said, according to TASS.Three Ukrainian servicemen were wounded and received medical care from the Russians, TASS said.The Ukrainian navy, on the other hand, 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.In the wake of the incident, Russia temporarily closed off the strait. However, it has since been reopened for civilian ships, according to TASS.Video from the scene showed a tanker blocking the waters under the bridge and fighter jets flying near the bridge.

Ukraine leader says he wants martial law

On Sunday, Poroshenko convened a military cabinet emergency meeting and later tweeted he would appeal to Parliament to declare martial law. "There are no red lines" for Russia, he said. "We consider such actions categorically unacceptable. And this aggression has already led to consequences."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.On Sunday, outgoing US Ambassador to the United Nations Nikki Haley confirmed that an emergency United Nations Security Council meeting has been called for Monday morning.Ukraine and the United States have been accusing Russia of interfering with international shipping in and out of the Kerch Strait for some months now.Russia and Ukraine have been locked in long-simmering conflict since the annexation of the Black Sea peninsula of Crimea by Russia in 2014, and a war with Russian-backed separatists in the country's east has claimed more than 10,000 lives. The incident at sea raises new tensions between the two countries. Russia has also backed separatists in Ukraine's eastern Donbas region.Russian state media has reported the confrontation is a provocation orchestrated by Ukraine and others in America to disrupt the forthcoming meeting between Russian President Vladimir Putin and President Donald Trump at the G20 summit in Buenos Aires this week.

CNN's Joshua Berlinger and Darran Simon contributed to this report.