Russia has opened fire on Ukrainian ships and captured three vessels in a major escalation of tensions off the coast of Crimea.
Three sailors have been wounded after the Ukrainian navy said two gunboats were hit by the strikes in the Black Sea.
Both countries have blamed each other for the incident.
The crisis began when Russia accused the Ukrainian ships of illegally entering its waters.
Russia placed a tanker under a bridge in the Kerch Strait to block the Ukrainian vessels from entering the Sea of Azov.
Ukraine said it had informed the Russians of its plan to move its ships through.
Ukrainian president Petro Poroshenko has called an emergency session of his war cabinet amid reports that martial law could be imposed.
The EU and NATO have called for restraint and urged Moscow to restore freedom of passage at the Kerch Strait.