MOSCOW (Reuters) -Six people, including one child, were killed on Friday in a Ukrainian missile attack on the town of Rylsk in Russia's Kursk region, the acting governor, Alexander Khinshtein, said.
Ten wounded people, including a 13-year-old, were taken to hospital with minor injuries, Khinshtein wrote on Telegram.
"What happened today is a huge tragedy for all of us," he said. "We grieve together with the families of the victims. No one will be left without support."
Those responsible would receive "well-deserved retribution", he said.
Russia's permanent representative to the United Nations, Vasily Nebenzya, later told a session of the Security Council that Moscow would be quick to respond to the "unambiguous move by the Kyiv regime".
"As you can well understand, our response to this targeted crime against peaceful Russian citizens will not be long in coming," he told the session.
Ukrainian authorities have not commented on the incident.
Khinshtein said Ukraine had fired U.S.-supplied HIMARS rockets, damaging several buildings including a school, recreation centre and private residences in Rylsk, some 16 miles (26 km) from the border with Ukraine's Sumy region.
An unconfirmed earlier report by the Mash Telegram channel, which is close to Russian law enforcement, put the death toll at seven.
The channel published unverified video showing damaged buildings and cars on fire in a city street.
Khinshtein, posting later on Telegram, said the attack had also disrupted heating and gas networks and work was under way to restore supplies to more than 80 residences.
Ukrainian troops still hold part of Kursk region after bursting across the border in a surprise incursion on Aug. 6. Russian President Vladimir Putin said during his annual press conference on Thursday that they would be expelled, but declined to set a date for when this would happen.
Khinshtein, who served as a State Duma deputy until he was appointed acting governor of Kursk by Putin earlier this month, accused Kyiv of deliberating targeting civilians in the strike.
Both Ukraine and Russia regularly accuse the other of attacking non-combatants, and both deny it.
Tensions between the warring sides have ratcheted up since Tuesday, when a top Russian general, Igor Kirillov, was killed by a bomb in Moscow. Ukraine's SBU intelligence service claimed responsibility for the assassination.