Russia accused Ukraine of carrying out a "terrorist attack" Saturday on civilians in the city of Belgorod, including using controversial cluster munitions in the strikes that killed at least 21 and wounded dozens more.
Moscow said the attack — which followed waves of rocket and drone attacks by Russian forces on Ukrainian cities — would "not go unpunished."
In an emergency meeting at the UN Security Council demanded by Russia, envoy Vasily Nebenzya claimed Kyiv targeted a sports center, an ice rink and a university.
"In order to increase the number of casualties of the terrorist attack they used cluster munitions," he said.
"[It was a] deliberate, indiscriminate attack against a civilian target."
Both Russian President Vladimir Putin and Ukraine's President Volodymyr Zelensky were due to give New Year's Eve speeches Sunday, as the conflict approaches its second anniversary in February.
The Belgorod attack came a day after Ukraine said a barrage of Russian missile strikes on several cities killed at least 39 people, wounding dozens more.
Fresh strikes Saturday caused more casualties in the northeastern city of Kharkiv, Ukrainian officials said.
Belgorod lies about 30 kilometers (19 miles) from the border with Ukraine and has been repeatedly struck by what Moscow says is indiscriminate shelling by Kyiv's forces.
Unverified footage showed a street strewn with debris and smoke billowing from burnt-out cars in the city's center.
AFP was not able to immediately verify the circumstances of the strike, one of the deadliest on Russian soil since Moscow launched hostilities against Ukraine in February 2022.
'Not go unpunished'
Russian authorities said the death toll included at least three children, with 17 children among the 110 wounded.
The Kremlin said Putin had been briefed on the incident, while the Russian Defense Ministry warned the strike would "not go unpunished."
Russia's Foreign Ministry, which has repeatedly denounced Western arms deliveries to Ukraine, accused the United States and Britain of "inciting the Kyiv regime to commit terrorist actions."
Ukrainian officials did not immediately comment.
In Ukraine, rescuers continued to search through rubble Saturday, a day after Russian strikes killed at least 39 people, one of the fiercest attacks since the early days of the conflict.
Schools, a maternity hospital, shopping arcades and blocks of flats were among the buildings hit in Friday's barrage, which triggered international condemnation.
"Work is still underway to eliminate the consequences of yesterday's Russian attack," Zelensky said.
Three more people were killed by Russian strikes across Ukraine on Saturday, local officials said.
And the prosecutor's office said Russian rocket attacks on Kharkiv Saturday evening had wounded 20 people after hitting a range of buildings including a hotel, a kindergarten, shops and restaurants.
The casualties included a British national, initially identified as a journalist, who was in fact a security adviser to a German media team, the statement added.
Jan. 1 will be declared a day of mourning in the capital Kyiv, where at least 17 people were killed, city officials said.
Russia's army said it had "carried out 50 group strikes and one massive strike" on military facilities in Ukraine over the past week, adding that "all targets were hit."
The United Nations condemned the attacks and said they must stop "immediately."
Missile 'entered' Polish airspace
Poland reported that a Russian missile briefly passed through its airspace during Friday's attacks.
After speaking to Polish President Andrzej Duda, NATO chief Jens Stoltenberg said the alliance "stands in solidarity" with Poland, adding: "NATO remains vigilant."
In the face of sustained Russian assaults, Ukraine is urging Western allies to maintain military support.
US President Joe Biden on Friday called on Congress to overcome its division to approve new aid for Ukraine, after Washington released its final package of weaponry under existing agreements.
"Unless Congress takes urgent action in the new year, we will not be able to continue sending the weapons and vital air defense systems Ukraine needs to protect its people," Biden said.
Britain announced it would send hundreds more air-defense missiles to Kyiv, after Prime Minister Rishi Sunak declared: "We must continue to stand with Ukraine -- for as long as it takes."
