The Russian authorities said on Saturday that a Ukrainian attack on the city of Belgorod had killed at least 22 people and injured nearly 110 others, in what would be the deadliest single assault against a Russian city since the start of the war nearly two years ago.

Russia’s Defense Ministry said in a statement that Ukraine had hit Belgorod — a regional center of around 330,000 residents about 25 miles north of the Ukrainian border — with two missiles and several rockets, adding that the strike was “indiscriminate.”

The ministry said that most of the rockets had been shot down, but that some debris had fallen on the city. The Ukrainian government has not officially commented on the Belgorod attack, and Russian claims could not be independently verified.

The attack seemed to be Ukraine’s response to a massive and deadly Russian air assault against its territory a day earlier, and another sign of Kyiv’s determination to bring the war to Moscow’s doorstep. In his overnight address on Friday, President Volodymyr Zelensky of Ukraine said that his country would continue to “work toward pushing the war back” to “where it came from — home to Russia.”

Saturday’s attack on Belgorod were quickly followed by what Ukrainian officials said were several Russian strikes against the Ukrainian city of Kharkiv, about 40 miles from Belgorod, in apparent cross-border retaliation.

The back-to-back assaults underscored how both Moscow and Kyiv remain willing to escalate a war that will most likely mark its two-year anniversary in February, despite Ukraine’s problems with securing Western funding, an increased sense of war fatigue in Russia and enormous casualties on both sides.