Russia has accused Ukraine of targeting civilian areas in one of its cities and using cluster munitions, during an emergency meeting of the UN Security Council requested by Moscow.
Russia said Ukraine attacked the city of Belgorod with missiles and rockets, killing at least 18 people and wounding dozens more.
"(It was a) deliberate, indiscriminate attack against a civilian target," said Russia's ambassador to the UN Vasily Nebenzya, claiming that Kyiv targeted a sports center, an ice rink and a university.
"UN Security Council members have an opportunity to do their duty and assess the damage done to a Russian city, Belgorod," he said, holding up a QR code linking to what he said was video of the attack's aftermath.
Ukrainian allies quickly retorted, saying Russia had unleashed the war.
The French envoy said Ukraine was simply defending itself under UN laws, while Moscow was "trampling" over the UN Charter.
Ukraine, which has been resisting a Russian invasion for nearly two years and earlier this week came under a huge Russian missile and drone assault, has not officially commented on the strike against Belgorod.
Belgorod lies about 30 kilometers from the border with Ukraine and has been repeatedly struck by what Moscow says is indiscriminate shelling by Kyiv's forces.
"As the war continues we will see more Ukrainian and Russian civilians killed," said UN assistant secretary-general Mohamed Khiari, warning there were "very real dangers of escalation and spillover of this war."
Unverified footage showed a street strewn with debris and smoke billowing from burnt-out cars in the city's center, while a large blast could be heard on dashcam footage posted on social media.
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.
Two Vilkha missiles and Czech-made rockets were used in the attack, Russia said.
Both Russian President Vladimir Putin and Ukraine's President Volodymyr Zelensky are due to speak tomorrow, New Year's Eve, as the conflict approaches its second anniversary in February.
The Kremlin said President Putin had been briefed on the incident, while the Russian defence ministry warned the strike would "not go unpunished".
Russia's foreign ministry, which has repeatedly railed against 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, the emergency services continued to search through rubble, 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 all among the buildings hit in yesterday's barrage, which triggered international condemnation.
Ukraine's military estimated Russia had launched 158 missiles and drones on Ukraine and 114 of them had been destroyed.
"Work is still underway to eliminate the consequences of yesterday's Russian attack," Mr Zelensky said.
New Year's Day will be declared a day of mourning in the capital Kyiv, where at least 16 people were killed, Mayor Vitali Klitschko said.
Three more people were killed by Russian strikes across Ukraine, local 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".
Ukraine Interior Minister Igor Klymenko said at least seven people were injured in the city of Kharkiv, including a British journalist.
The United Nations condemned the attacks and said they must stop "immediately".
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.
Ukraine presidential aide Andriy Yermak said Kyiv needed "more support and strength to stop this terror".
US President Joe Biden yesterday 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 defence systems Ukraine needs to protect its people," Biden said.
"Congress must step up and act without any further delay."
Britain announced it would send hundreds more air-defence missiles to Kyiv, after Prime Minister Rishi Sunak declared: "We must continue to stand with Ukraine, for as long as it takes."
