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 injured.
Each country blames the other for the incident. On Monday Ukrainian MPs are due to vote on declaring martial law.
The crisis began when Russia accused the Ukrainian ships of illegally entering its waters.
The Russians placed a tanker under a bridge in the Kerch Strait - the only access to the Sea of Azov, which is shared between the two countries.
During a meeting of Ukraine's National Security and Defence Council, President Petro Poroshenko described the Russian actions as "unprovoked and crazy".
Russia has requested an emergency meeting of the UN Security Council, which US ambassador to the UN Nikki Haley says has been called for 11:00 New York time (16:00 GMT) on Monday.
Tensions have recently risen in the Black Sea and the Sea of Azov off the Crimean peninsula - annexed by Russia in 2014.
How did the crisis unfold?
In the morning, 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.
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.
Russia's FSB later confirmed that one of its patrol boats had used force to seize the three Ukrainian vessels but said only three sailors had been wounded.
Ukraine said it had informed the Russians of its plan to move its ships through the sea to Mariupol.
Analysis by Steven Rosenberg, BBC News, Moscow
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 use of force by Russia to seize Ukrainian vessels - with casualties - is a major escalation. But you won't hear Moscow taking the blame.
Under President Vladimir Putin, when Russia has used force before, its line of defence has always been: "We didn't start it." That goes for the Russia-Georgia War of 2008, and the appearance of "Little Green Men" (Russian special forces) in Crimea in 2014, which preceded Moscow's annexation of the peninsula.
So, expect Moscow to pin the blame for what happened on Sunday and for whatever happens next on President Poroshenko's government.
What has the international response been?
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".
What's the background to this?
The shallow Sea of Azov lies east of Crimea, and south of the Ukrainian regions partially seized by pro-Russian separatists.
The two Ukrainian ports on its northern shore - Berdyansk and Mariupol - are key to exporting grain and produce such as steel, also for importing coal.
The 2003 treaty between Ukraine and Russia guaranteed free navigation to both countries' vessels.
But Russia has recently begun inspecting ships going to or from Ukrainian ports. Earlier this month, the EU warned it would take "targeted measures" to address the issue.
The inspections began soon after Ukraine detained a fishing vessel from Crimea in March. Moscow says they are necessary for security reasons, pointing to a potential threat to the bridge from Ukrainian radicals.
More than 10,000 people have been killed in the eastern Donetsk and Luhansk regions since separatists moved against the Ukrainian state in April 2014.
Ukraine and the West accuse Russia of sending its troops to the region and arming the separatists.
Moscow denies this but says that Russian volunteers are helping the rebels.