"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.

European Commission chief Ursula von der Leyen called the attacks on Kyiv and other Ukrainian cities "vicious".

"Russia once again has shown to the world what it stands for -- it is terror and brutality. Those who are responsible have to be held accountable," she said in a statement.

The EU's foreign policy chief, Josep Borrell, said on Twitter he was "deeply shocked" by Russia's attacks on civilians.

"Such acts have no place in (the) 21st century. I condemn them in the strongest possible terms," he said.

He added that "additional military support from the EU is on its way" for Ukraine, referring to a new 500-million-euro ($486-million) tranche of funding the EU is looking to agree for Kyiv's military purchases from bloc members.

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.