Separatist rebels attacked a Ukrainian naval vessel in the Azov Sea by firing artillery from the shore, and a Ukrainian military spokesman said a rescue operation was under way.
Spokesman Andriy Lysenko said the vessel was a naval cutter. There was no information on the number of people on board.
The pro-Russian rebels claimed responsibility for the attack.
The separatists have been fighting government forces since April in a conflict that has killed some 2,600 people, but this was the first naval attack of the war.
It came after rebel forces opened a new front in the fighting last week, breaking through to the Azov Sea in Ukraine's southeast.
"The militia have dealt the enemy their first naval defeat," Igor Strelkov, a former separatist military commander, said on the social media network VKontakte.
