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.