Russian President Vladimir Putin said on Monday that Ukraine had carried out "terrorist acts" against Russia and pledged to react forcefully if they continued.
In televised remarks, Putin said Moscow had launched long-range missile attacks against Ukraine's energy, military and communications infrastructure on Monday in retaliation for an attack on a vital bridge linking Russia to the annexed Crimean peninsula over the weekend.
"It is obvious that the Ukrainian secret services ordered, organized and carried out the terrorist attack aimed at destroying Russia's critical civilian infrastructure," Putin said of the bridge explosion.
Ukrainian officials were exultant after the blast but Kyiv has not claimed responsibility.
Cities across Ukraine were left without power or water and several were killed in Russian missile strikes on more than a dozen Ukrainian cities on Monday morning.
Putin said Russia would respond "harshly" to any further attacks by Ukraine.
"If attempts continue to carry out terrorist acts on our territory, Russia's responses will be harsh and in their scale will correspond to the level of threats created for the Russian Federation. No one should have any doubt about that," Putin said.
Earlier, the Kremlin said the explosions that rocked Kyiv on Monday were part of what it calls its "special military operation".
"Everything is happening within the framework of the special military operation," Kremlin spokesman Dmitry Peskov said.
Peskov said Russia had launched an investigation into the explosion on the bridge on Saturday morning, which killed three people and was seen as a highly symbolic blow against Russia and Putin.
