Russia said Saturday that a Ukrainian strike on the city of Belgorod that it said killed at least 14 people "would not go unpunished".
"This offence will not go unpunished," Russia's defence ministry said in a statement.
Russia says 14 killed in Ukrainian strike on Belgorod
Russia said Saturday that a Ukrainian strike killed at least 14 people and injured 108 in the city of Belgorod, a day after Russian strikes killed dozens in Ukraine.
Belgorod lies about 30 kilometres (19 miles) from the border with Ukraine and has been repeatedly targeted by what Russia 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 blast.
"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.
President Vladimir Putin has been briefed on the incident, the Kremlin said, while the Russian defence ministry warned the attack would "not go unpunished".
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.
- 'Stop this terror' -
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".
Poland reported that a Russian missile briefly passed through its airspace.
"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."