Ukraine Parliament to vote on martial law

Ukrainian lawmakers were set to vote on whether to implement martial law after Russia seized three Ukrainian navy ships and detained 24 sailors in a key waterway that holds strategic importance for both countries.

The incident on Sunday marked a major escalation in tensions between the two former Soviet Republics. NATO has called for calm, and an emergency United Nations Security Council meeting will be convened Monday to discuss the issue.

Ukraine said the small gunboats Berdyansk and Nikopol and the tugboat Yany Kapu were attacked by Russian forces after entering the Kerch Strait, en route to the city of Mariupol, on Sunday.

Video of the incident released by Ukrainian officials appeared to show a Russian ship ramming the Ukrainian tugboat.

All 24 sailors aboard the three boats were detained by Russia, state news agency RIA reported on Monday, citing sources in Crimea's security agencies. Three of the sailors were now in hospital, it added.

However, according to Ukraine's navy, six of its sailors were injured in the incident. It added that the tugboat and the Berdyansk were disabled and towed away by the Russian navy. The undamaged Nikopol is also under Russian control, Ukraine said.

The Kerch Strait -- a shallow, narrow stretch of water just 2-3miles wide at one point -- connects the Azov Sea with the Black Sea and runs between the Crimean Peninsula and Russia. It is an important economic lifeline for Ukraine, as it allows its ships to access the Black Sea.

It's also the closest point of access for Russia to Crimea, a peninsula Moscow annexed from Ukraine in 2014. A Russian-built bridge over the Kerch Strait was opened in May this year.

NATO Secretary General Jens Stoltenberg pledged the alliance's "full support for Ukraine's territorial integrity" following a phone call with Ukrainian President Petro Poroshenko on Monday morning. An extraordinary meeting of NATO-Ukraine ambassadors is planned in Brussels later Monday.