Ukrainian Defense Ministry spokesman Vladyslav Seleznyov has posted a statement on his Facebook site that Russian forces were in the process of taking control of the last Ukrainian warship in Crimea.
Seleznyov said late Tuesday "there were several explosions from the direction of the minesweeper Cherkasy in the Donuzlav Bay."
Seleznyov said Mi35 helicopters were flying over the ship as troops rushed aboard.
Ukrainian warships along the Crimean Peninsula have been seized one-by-one for nearly a week.
Russian naval vessels had been blocking them from leaving their docks.