Series of blasts were heard in the early hours of Wednesday in three Russian provinces bordering Ukraine, authorities said, and an ammunition depot in the Belgorod province caught fire around the same time.
Belgorod regional governor Vyacheslav Gladkov said a fire at the ammunition depot near the Staraya Nelidovka village had been extinguished and no civilians have been injured.
Russia this month accused Ukraine of attacking a fuel depot in Belgorod with helicopters and opening fire on several villages in the province.
The Belgorod province borders Ukraine's Luhansk, Sumy and Kharkiv regions, all of which have seen heavy fighting since Russia invaded Ukraine two months ago.
Separately, Roman Starovoyt, the governor of Russia's Kursk province, which also borders Ukraine, said that explosions had been heard in Kursk city early on Wednesday which were most likely the sounds of air defence systems firing.
In Voronezh, the administrative centre of another province adjacent to Ukraine, Russia's TASS news agency cited an emergencies ministry official as saying that two blasts had been heard and the authorities were investigating.
Russia sent tens of thousands of troops into Ukraine on February 24 in what it called a "special operation" to degrade its military capabilities and root out what it calls dangerous nationalists. Ukrainian forces have mounted stiff resistance and the West has imposed sweeping sanctions on Russia in an effort to force it to withdraw its forces.