President Vladimir Putin said Monday that Russia's response to any further Ukrainian attacks would be "severe", after Moscow's forces carried out retaliatory missile strikes across Ukraine.
"It was not possible to leave (Ukrainian attacks) unanswered. If attempts at terrorist attacks continue, the response from Russia will be severe and correspond to the level of threat," Putin said at the start of a televised meeting of his security council.
"Let there be no doubt about it," Putin said.
His remarks come after a huge blast on Saturday damaged a key bridge in Crimea, Putin's flagship project and a vital transport link between Russia and the peninsula Moscow annexed in 2014.
In response to the attack, Putin said Russia carried out "a massive strike with high-precision, long-range weapons... on energy, military command and communications facilities in Ukraine".
Russia's defence ministry said in a statement that the series of strikes it fired "have achieved their objectives. All targets have been hit".
Putin also accused Ukraine of launching three attacks on the Kursk nuclear power plant in Russia, about 85 kilometres (53 miles) from the Ukrainian border and of attempting to hit the TurkStream gas pipeline running from Russia to Turkey under the Black Sea.
Ukraine's capital Kyiv was hit by multiple Russian strikes early on Monday -- the first since late June, AFP journalists witnessed.
Russia's former president Dmitry Medvedev said the strikes were only the "first episode" and that Russia's goal should be "the complete dismantling of Ukraine's political regime".
Russia also launched attacks on several other cities across Ukraine, particularly targeting energy infrastructure.
Electricity cuts were reported in several regions, including Ukraine's second city Kharkiv and its surrounding region, plus the northeastern Sumy region, Zhytomyr region in the north and Khmelnitskyi region in the west.
Meanwhile, another report adds from Brussels: The EU believes Russia's missile attacks on civilians in Ukraine "amounts to a war crime," a spokesman for the bloc's foreign policy chief Josep Borrell said Monday.
"Indiscriminately targeting people in a cowardly, heinous hail of missiles on civilian targets is indeed a further escalation," the spokesman, Peter Stano, said.
"The European Union condemns in the strongest possible terms these heinous attacks on the civilians and civilian infrastructure.... This is something which is against international humanitarian law and this indiscriminate targeting of civilians amounts to a war crime," he said.
Asked about Russia's ally Belarus agreeing to deploy a "regional grouping" of Russian and Belarusian troops to an unspecified theatre, Stano warned Minsk to "refrain" from further helping Moscow in Ukraine.
"We don't have the details (on the joint deployment) but if this proceeds, this will be yet another escalation" of the "illegal war" in Ukraine," Stano said.
"And this will not be unanswered from the side of the European Union," he warned.
The EU has already imposed sanctions on Belarus for providing its territory for Russia to launch part of its invasion of Ukraine, and has said it stands ready to add to them if Minsk provides more help to Moscow.
The European bloc has imposed eight rounds of sanctions on Russia for its war in Ukraine, severely hitting its economy.
