Volodymyr Zelensky has accused Russia of “crimes against humanity” in a UN address after a barrage of Russian missiles plunged Ukrainian cities into darkness and killed 10 people.
Mr Zelensky urged the UN Security Council to act against Russia over its relentless air strikes on civilian infrastructure.
He estimated that on Wednesday alone some 70 missiles were fired, forcing nuclear power plants to close and cutting off energy and water supply in many places.
“Today is just one day, but we have received 70 missiles. That's the Russian formula of terror,” he said.
“This is all against our energy infrastructure. Hospitals, schools, transport, residential districts all suffered.”
Ukraine was waiting to see "a very firm reaction" to Wednesday's air strikes from the world, he added.
The council is unlikely to take any action in response, as Russia is a member of the council with veto power.
The US ambassador to the UN, Linda Thomas-Greenfield, said Vladimir Putin was “weaponising winter” with his strikes on energy infrastructure.
“Having struggled on the battlefield, Moscow is now adopting a cowardly and inhumane strategy that punishes Ukrainian men, women and children," she said.
Neighbouring Moldova also experienced blackouts on Wednesday, but was not directly hit by missiles.
In the southern Zaporizhzhia region, a newborn baby was killed when a missile hit a maternity unit, emergency services said.
Russia's UN ambassador, Vasily Nebenzya, responded by complaining that it was against council rules for Mr Zelensky to appear via video.
French President Emmanuel Macron tweeted on Wednesday that Russia's strikes against civilian infrastructure constituted a war crime, following similar comments from the US.
Earlier the European Parliament designated Russia a “state sponsor of terrorism” over the war in Ukraine.
The temperature was minus 3.4 Celsius in Kyiv early on Thursday and 70 per cent of city remained in blackout, the mayor said.
Since October, Russia has acknowledged targeting Ukraine's civilian energy grid far from front lines as a Ukrainian counteroffensive has recaptured territory from Russian forces in the east and south.
Moscow says the aim of its missile strikes is to weaken Ukraine's ability to fight and push it to negotiate.