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.

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