Belgorod governor Vyacheslav Gladkov said a Ukrainian army "sabotage group" had entered Russian territory in the Graivoron district, which borders Ukraine.
Hromadske quoted military intelligence spokesperson Andriy Yusov as saying the Freedom of Russia Legion and Russian Volunteer Corps were responsible for the incursion. It did not make clear whether he provided any evidence for the assertion.
"Responsibility for these events was taken by citizens of (Russia), particularly the RDK and the 'Freedom of Russia' Legion," he said, using the acronym for the Russian Volunteer Corps.
"I think we all can only welcome the decisive actions of opposition-minded Russian citizens, who are ready for an armed struggle against the criminal regime of (President) Vladimir Putin."
Yusov said the Belgorod operation would create a "security zone" to protect Ukrainians from cross-border attacks by Russia.
(Reporting by Felix Hoske and Dan Peleschuk, Editing by Timothy Heritage)
