Russia said on Saturday it would punish Ukraine for using a sea drone to attack a civilian tanker near the Kerch Strait in what it said was a "terrorist act" that threatened the lives of the crew and risked "a large-scale environmental disaster".
Both sides said on Saturday that a Ukrainian sea drone full of explosives had struck a Russian fuel tanker overnight near a bridge linking Russia to annexed Crimea, the second such attack in 24 hours.
Russian Foreign Ministry spokeswoman Maria Zakharova said that Moscow would retaliate.
"The Kyiv regime, meeting no condemnation from Western countries and international organisations, is actively applying new terrorist methods, this time in the waters of the Black Sea," Zakharova said in a statement.
"There can be no justification for such barbaric actions, they will not go unanswered and their authors and perpetrators will inevitably be punished."
Former Russian President Dmitry Medvedev earlier on Saturday suggested Moscow would launch more strikes against Ukrainian ports in response to Kyiv's attacks on Russian ships in the Black Sea, and threatened to hand Ukraine "an ecological catastrophe".