The attack in Belgorod, Russia, left 24 dead and 108 injured, according to Russian authorities.
Moscow says Kiev is responsible, but Ukraine has so far remained silent.
These bombings killed 24 people and injured 108 others, according to a new toll announced by the region's governor, Vyacheslav Gladkov.
Kiev regularly carries out attacks in Russian territory, in particular using drones, but this strike is the deadliest for civilians in Russia since the start of the conflict in February 2022.
In response to this terrorist act, Russian armed forces struck decision-making centers and military installations in Kharkiv, the Russian Defense Ministry said on Sunday.
The governor of the targeted Ukrainian region, Oleg Sinegoubov, nevertheless assured that rockets had hit a hotel, residential buildings, clinics and hospitals on Saturday evening, leaving 28 injured.
Among them, two teenagers and a Briton, who was the security adviser to a team of German journalists, according to Ukrainian authorities.
The Russia admitted to having targeted a former hotel complex, the Kharkiv Palace, but assured that there were members of military intelligence and the Ukrainian armed forces involved in the attack on Belgorod, as well as foreign mercenaries.
The Kharkiv Palace hotel was hit by Russian strikes.
Moscow still denies targeting civilian targets in Ukraine.
Kharkiv Mayor Igor Terekhov said drone attacks had also damaged cafes, residential buildings and offices, without reporting any casualties.
On New Year's Eve, the Russians want to intimidate our city, but we are not afraid, he added.
For its part, the Ukrainian Air Force claimed that six Russian guided missiles had targeted Kharkiv.
It also said said to have shot down 21 of the 49 Iranian-made Shahed drones launched overnight by Russia towards its territory. an escalation of violence between Russia and Ukraine.
On Friday, Ukraine was devastated by a missile attack that it said was the most massive since the start of the conflict, excluding the very first days.
President Volodymyr Zelensky said Saturday afternoon that 39 people had died across the country, but more deaths have since been announced.
Nearly 120 towns and villages were hit by Russian strikes on Friday, Ukrainian President Volodymyr Zelensky said.
< p class="StyledBodyHtmlParagraph-sc-48221190-4 hnvfyV">The governor of the Dnipropetrovsk region, Serguiï Lyssak, declared on Sunday that one of the injured had died in hospital, bringing the death toll for his area to seven.
< p class="StyledBodyHtmlParagraph-sc-48221190-4 hnvfyV">In Kiev, the State Emergency Service said it found 23 bodies in the rubble after Friday's attack.
On Saturday, during an emergency meeting of the UN Security Council on the Belgorod strike, Russian Ambassador to the United Nations Vasily Nebenzia accused Kiev of ;an indiscriminate and deliberate attack on a civilian target.
British Representative Thomas Phipps retorted that if Russia wants to blame someone for the deaths of Russians in this war, it should start with President Putin.
On both the Russian and Ukrainian sides, New Year celebrations will take place under high tension on Sunday evening, after the strikes of the last few days.
The governor of the Russian region of Kursk, Roman Starovoit, invited residents to stay at home, due to the turbulent situation.
His counterpart in the Voronezh region, Alexander Gusev, indicated that he had canceled the festive programs, out of solidarity with the residents of Belgorod.