Ukraine said that Russia had pounded the country with hundreds of drones and missiles overnight for hours, killing at least four people in the capital alone.
Neighbouring Poland scrambled jets to secure its airspace in the wake of the barrage, after NATO accused Moscow of being behind a series of violations of the defence alliance's airspace.
Diplomatic efforts to stop the war, now nearing its fourth year, have faltered, while Russia has vowed to press on with its offensive.
"Moscow wants to continue fighting and killing and deserves only the harshest pressure from the world," Ukrainian President Volodymyr Zelenskyy said following the strikes, which lasted for 12 hours.
"The Kremlin benefits from continuing this war and terror as long as there are profits from energy sales," he added, urging stronger measures against Russia from Kyiv's allies.
The Ukrainian leader vowed his country would target Russia's ability to fund the war and force Moscow to diplomacy.
He posted a video of residential buildings caught up in flames, with emergency services putting down fires and rescuing people from shattered buildings.
Besides the dead, which included a 12-year-old girl, more than 40 people were wounded in the Zaporizhzhia, Odesa, Sumy, Cherkasy and Mykolaiv regions, Ukrainian authorities said.
More victims could be uncovered as rescuers went about their jobs, they warned.
Apart from residential buildings, a cardiology centre and a kindergarten were among the targets across the country, Ukraine's foreign ministry said.
Andriy Yermak, the head of Ukraine's presidential office, accused Moscow of waging a "war against civilians", urging greater action from Kyiv's Western allies.
"There will be a response to these actions. But the West's economic blows against Russia must also be stronger," Yermak said.
Russia rained down 643 drones and missiles, including glide bombs, across eastern, central and southern Ukraine, Ukraine's general staff said.
The attacks came after Russia warned NATO against taking sterner action in response to alleged incursions into airspace covered by the military alliance.
'They will regret it'
Poland's armed forces said on X that Warsaw had scrambled fighter jets in its airspace and put ground-based air defence systems on high alert in response to Russia's strikes.
In recent weeks, several European countries have accused Russia of violating their airspace with drones and fighter jets, in what NATO sees as a test of its resolve.
Russia has denied that it is responsible for the incursions or that it plans to attack any NATO nation.
In an address at the UN General Assembly in New York on Saturday, Moscow's foreign minister Sergei Lavrov warned that "any aggression against my country will be met with a decisive response".
Speaking later to reporters, Lavrov said that if any country downs objects still within Russian airspace, "they will very much regret it".
Zelenskyy said Ukraine received a US-made Patriot air defence system from Israel, and two more were expected to arrive soon, after he returned from his own trip to New York, where he also addressed the UN.
While initially neutral in the conflict, Israel's ties with Moscow have cooled as Russia has drifted closer to Iran and condemned Israel's war in Gaza.
US and Ukrainian delegations will hold several meetings in the coming weeks on weapons purchases, as Kyiv provided Washington with a list of weapons Ukraine wanted, part of a plan to purchase US$90 billion ($137 billion) worth of arms from Washington, according to Zelenskyy.
Kyiv and Moscow also said on Sunday AEST that the Russian-occupied Zaporizhzhia nuclear power plant — Europe's largest — had been off the grid for four days, stoking fears of a potential nuclear incident.
