Ukrainian naval vessels have been fired on by Russia off the coast of Crimea, with a number of ships also seized.
A tug and two gunboats were captured by Russian forces in a massive escalation of tensions between the two countries. A number of casualties were also reported in the incident.
Russia has accused Ukraine of illegally entering its waters, after it illegally annexed Crimea from the country in 2014.
Russia has also blocked Ukrainian access to the Sea of Azov by placing a tanker in the Kerch Strait, preventing vessels from docking in Berdyansk and Mariupol – ports vital for exporting grain and steel, and importing coal.
Ukrainian President Petro Poroshenko has called the Russian actions “unprovoked and crazy”.
Poroshenko has also planned a vote in parliament for Monday on declaring martial law.
An emergency meeting of the UN Security Council, requested by Russia, is set to take place at 11am on Monday (4pm GMT).
This latest crisis is part of an ongoing conflict in eastern Ukraine, where Ukrainian forces are fighting pro-Russian separatists.
The Sea of Azov lies to the east of Crimea and to the south of regions partially occupied by the separatists. Under a 2003 treaty, the sea and the Kerch Strait into the sea is shared territorial water between Ukraine and Russia.
However, Russia has recently begun inspecting vessels sailing to and from Ukrainian ports operating in the area.
NATO reaffirmed its support for Ukraine’s territorial integrity and called upon Russia to “ensure unhindered access to Ukrainian ports in the Azov Sea”.