Local officials in Russian districts bordering Ukraine reported overnight blasts early on Wednesday, one of them appearing to strike an ammunition depot, raising the spectre of broader conflict spilling beyond Ukraine’s borders.

Explosions were reported in Belgorod, Russia, less than 20 miles from the border with Ukraine, as well as in Kursk and Voronezh, deeper into Russian territory, according to local officials and the state-run press agency Tass. In Belgorod, an ammunition depot was on fire but no homes had been hit and there were no civilian casualties, the regional governor, Vyacheslav Gladkov wrote on his Telegram channel.

It was not clear what caused the blasts. In Kursk, less than 100 miles from the border and Voronezh, nearly 200 miles from Ukraine, air defence systems had reportedly been activated. Ukraine’s defence ministry has declined to discuss reports of attacks on Russian soil.

New York Times News Service