A Ukrainian naval vessel was in flames in the Azov Sea after being attacked by artillery from pro-Russian separatists along the shore.
Ukrainian military spokesman Andriy Lysenko said an emergency operation is under way to help rescue crew members.
Lysenko said the vessel is a naval cutter but it is not known how many people were onboard.
Pro-Russian insurgents claimed responsibility for the attack on social media websites.
Igor Strelkov, a former separatist military commander, said on VKontakte that the separatists have "dealt the enemy their first naval defeat."
Rebels backed by Russian forces seized the Ukrainian town of Novoazovsk and nearby territory last week in a major assault that led to gains in many parts of eastern Ukraine.
Based on reporting by Reuters, AP