"If something like this happens, Russia will give a tough response using more powerful weapons," warned a prominent Russian lawmaker.

A close ally of President Vladimir Putin warned Western governments on Thursday that a nuclear war would ensue if they gave the green light for Ukraine to use long-range Western weapons to strike targets deep inside Russia.

Russian officials delivered dissonant statements in the past week about how Moscow would respond to Western weapons striking deep inside its territory, and whether such attacks will happen at all. Russian diplomats played on the now-familiar threat of a nuclear response.

A dozen other people were injured, some seriously, in the attack in the city of Sumy, which has come under increased bombardment since Ukraine’s offensive into Russia’s nearby Kursk region.

Ukraine's air force said on Thursday it had shot down all 42 drones and one of four missiles used by Russia during overnight attacks. Air defences went into operation in nine Ukrainian regions, the air force said in a statement on the Telegram messaging app nearly 31 months after Russia's full-scale invasion.