Russia accused Ukraine of targeting the city of Belgorod with missiles and rockets on Saturday, killing at least 14 people and wounding dozens more in an attack Moscow warned would "not go unpunished".
Belgorod lies about 30 kilometres (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 debris strewn across the street and smoke billowing from burnt-out cars in the city's centre, while a large blast can be heard in 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. It has requested a UN Security Council meeting for 2000 GMT.
"According to updated information, 12 adults and two children were killed in Belgorod. Another 108 people, including 15 children, were injured," Russia's emergencies ministry said.
The Kremlin said President Vladimir 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.
- 'Work underway' -
The attack came as rescuers in Ukraine continued to search through rubble Saturday after Russian strikes killed at least 39 people the day before, in 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 Friday'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.
Air force spokesman Yuriy Ignat told AFP that this was a "record number" of missiles and "the most massive missile attack" of the conflict, excluding the early days of constant bombardment.
"Work is still underway to eliminate the consequences of yesterday's Russian attack," Ukrainian President Volodymyr Zelensky 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.
Three more people were killed by Russian strikes across Ukraine on Saturday, 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".
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.
"Everything indicates that a Russian missile entered Polish airspace... It also left," said General Wieslaw Kukula, chief of the general staff of the Polish armed forces.
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 called on Congress to overcome its division to approve new aid for Ukraine, after Washington released its final package of weaponry under existing agreements still to be renewed.
"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 that "We must continue to stand with Ukraine -- for as long as it takes."