Russia said on Monday it was battling a cross-border incursion by saboteurs who burst through the frontier from Ukraine, in what appeared to be one of the biggest attacks of its kind since the war began last year.
The governor of Russia's Belgorod region said a Ukrainian "sabotage group" had entered Russian territory in the Graivoron district bordering Ukraine, and was being repelled.
But the Ukrainian outlet Hromadske cited Ukrainian military intelligence as saying two armed Russian opposition groups, the Liberty of Russia Legion and the Russian Volunteer Corps (RVC), both consisting of Russian citizens, had carried out the attack.
Kremlin spokesman Dmitry Peskov said Russian President Vladimir Putin had been informed and that work was under way to drive out the "saboteurs", the state-run RIA Novosti news agency reported.
A senior aide to Ukrainian President Volodymyr Zelenskiy said Kyiv had nothing to do with the incursion, putting it down to Russia's emerging "violent resistance movement".
Belgorod governor Vyacheslav Gladkov said on Telegram that the Russian army, border guards, presidential guards and the FSB security service were involved in the operation. He said at least six people had been wounded and three houses and an administrative building damaged.
The Telegram channel Baza, which has links to Russia's security services, said there were indications of fighting in three settlements along the main road leading into Russia. The "Open Belgorod" Telegram channel said power and water had been cut off to several villages.
Reuters was unable to verify the reports and the Ukrainian military was not immediately available for comment.
A group calling itself the Liberty of Russia Legion - a Ukraine-based Russian militia led by Russian opposition figure Ilya Ponomarev that says it is working inside Russia for Putin's overthrow - said on Twitter it had "completely liberated" the border town of Kozinka. It said forward units had reached the district centre of Graivoron, further east.
"Moving on. Russia will be free!" it wrote.
In a written statement to Reuters, senior Zelenskiy aide Mykhailo Podolyak agreed with Ukrainian military intelligence.
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