Russia said 20 people including two children had been killed and 111 injured in "indiscriminate" Ukrainian strikes on the Russian provincial capital of Belgorod on Saturday, and vowed to retaliate.
Two Vilkha missiles and Czech-made rockets were used in the attack, Russia said. It requested a UN Security Council meeting for later on Saturday.
“This crime will not go unpunished,” the Foreign Ministry said in a statement about the strike on the city about 30 kilometres north of Russia’s border with Ukraine.
The Belgorod region has, like other Russian border zones, suffered shelling and drone attacks all year that authorities have blamed on Ukraine.
Regional governor Vyacheslav Gladkov said a residential area had been hit and urged all residents to move to air raid shelters as sirens sounded.
Images posted by the state-run RIA news agency showed at least three cars on fire, and other images posted online showed black smoke rising from the city, Reuters reported.
Two residents told Reuters they had seen air defence missiles rising into the sky followed by explosions in the air and then louder blasts.
The Kommersant newspaper cited a source close to the Russian Investigative Committee as saying missiles fired from a multiple rocket launchers in Ukraine's Kharkiv region had hit a skating rink in the central Cathedral Square, a shopping centre, residential buildings and a car.
No official comment was immediately available from Kyiv, which rarely takes responsibility for attacks inside its neighbour, according to Reuters.
But the Ukrainian news outlet RBC-Ukraine quoted sources as saying Ukrainian forces had struck military targets in Belgorod in response to Russian bombardment of Ukrainian cities the previous day.
Within hours of the attack on Belgorod, Ukrainian authorities said two Russian S-300 missiles hit the centre of Kharkiv, Ukraine's second city, injuring 21 people including two boys aged 16 and 14.
One missile hit the Kharkiv Palace Hotel and another an apartment building. Officials said a medical institution and other civilian infrastructure were also damaged.
The attack came a day after Russian forces launched a barrage of air strikes across Ukraine in one of the deadliest attacks since the conflict began last year.
Ukraine's military estimated Russia had launched 158 missiles and drones on Ukraine and 114 of them had been destroyed.
Air force spokesman Yuriy Ignat told AFP that this was a “record number” of missiles and “the most massive missile attack” of the war, excluding the early days of constant bombardment.
Russia tried to overwhelm Ukraine's air defences across most major cities, launching a wave of Shahed attack drones followed by missiles of numerous types fired from planes and from Russian-controlled territory.
At least 39 people were killed in Friday's strikes, Ukrainian President Volodymyr Zelenskyy said.
“Work is still under way to eliminate the consequences of yesterday's Russian attack,” he wrote in a post on social media Saturday.
“In total, 159 people were injured in this terrorist attack. Unfortunately, 39 of them have been killed so far,” he said.
January 1 will be declared a day of mourning in the capital Kyiv, where at least 16 people were killed, Mayor Vitali Klitschko said.
The Russian attack brought more waves of international condemnation with US President Joe Biden urging Congress to overcome its divisions and approve to aid for Ukraine.
Agencies contributed to this report.
