(Reuters) -The governor of Russia's Belgorod region said on Monday that a Ukrainian "sabotage group" had entered Russian territory in the Graivoron district bordering Ukraine and was being repelled by Russian security forces.
Unverified reports of attacks well past midnight on the main town of the region bordering Ukraine were posted on social media. Reports on some channels said the local headquarters of the interior ministry and FSB security service had been targeted.
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), were carrying out the attack.
Hromadske said both were composed of Russian nationals committed to fighting the Kremlin authorities. Ukrainian presidential adviser Mykhailo Podolyak tweeted that Kyiv was "watching the events in the Belgorod region" but was not involved.
The Liberty of Russia Legion is a Ukraine-based Russian militia led by Russian opposition figure Ilya Ponomarev that says it is working inside Russia for Putin's overthrow.
It said on Twitter it had "completely liberated" the border town of Kozinka and forward units had reached the district centre of Graivoron, further east.
"Moving on. Russia will be free!" it wrote.
Belgorod regional Governor Vyacheslav Gladkov imposed a "counter-terrorism regime" allowing authorities greater powers to clamp down on people's movement and communications.
In a late-night post on Telegram, Gladkov said that in two separate attacks houses and administrative buildings where damaged in two towns in the region, Borisovka and Graivoron.
Telegram channels monitoring Russia's military activity, including the blog Rybar, with more than a million subscribers, said buildings housing the Interior Ministry and the FSB security service had come under attack in the region's main town, also known as Belgorod.
Gladkov made no mention of the alleged attack on Belgorod. Reuters was not able to independently verify the situation.
The Telegram channel Baza, which has links to Russia's security services, had earlier published aerial footage apparently showing a Ukrainian armoured vehicle advancing on the Graivoron border checkpoint.
Kremlin spokesperson Dmitry Peskov said 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.
In Telegram postings earlier in the day, Gladkov said the Russian army, border guards, presidential guard and the FSB were in the operation. He said at least eight people had been wounded and three houses and an administrative building damaged.
In a briefing streamed on social media, he said many residents had left, either in buses or their own vehicles, and that he had imposed a "counter-terrorist operation" regime.
Under expanded powers, authorities were authorised to limit activities and movement and to suspend or restrict communication services including mobile networks and the internet.
VIDEO DESCRIBES CAPTURED VEHICLE, SOLDIERS
The Russia Volunteer Corps published video footage late on Monday which showed what it said was a fighter inspecting a captured armoured vehicle. Another video showed what it said were fighters operating an armoured vehicle on a country road.
Other videos posted on Russian and Ukrainian social media channels showed pictures and video of what were described as captured Russian servicemen and their identity documents.
Baza 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.
The Liberty of Russia Legion released a video showing five armed fighters.
"We are Russians, like you. We are people like you. We want our children to grow up in peace," one said, facing the camera. "It is time to put an end to the dictatorship of the Kremlin."
Hromadske quoted Ukrainian military intelligence spokesperson Andriy Yusov as saying the operation would create a "security zone" to protect Ukrainians from attacks by Russia.
The Kremlin said the incursion aimed to distract attention from the eastern Ukrainian town of Bakhmut, which Russian forces claim to have captured in its entirety after more than nine months of attritional fighting.
"We understand perfectly well the goal of such a diversion - to divert attention from the Bakhmut direction and minimise the political effect of Bakhmut's loss for the Ukrainian side," Peskov was quoted as saying.
Early in March, the FSB reported an incursion from Ukraine into Russia's Bryansk region.
In videos circulating online at the time, armed men saying they belonged to the RVC said they had crossed the border to fight what they called "the bloody Putinite and Kremlin regime".
The RVC was founded last August by Denis Kapustin, a Ukraine-based Russian nationalist, and announced on May 17 that it was joining forces with the Liberty of Russia Legion, which calls itself the Freedom of Russia Legion in English.
The RVC says it has made at least three incursions into Bryansk region since March.
(additional reporting by Peter Graff, Agnieszka Pikulicka-Wilczewska, Ron Popeski; Writing by Kevin Liffey; Editing by Grant McCool and Stephen Coates)