ITV News Correspondent John Ray reports on the incursion into Russian territory by anti-Putin paramilitary groups
Ukrainian military intelligence has said 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.
A Ukrainian official acknowledged that the units had carried out an operation in the area but insisted they were acting independently.
"We can confirm that this operation was carried out by Russian citizens," Andriy Yusov, a representative of Ukraine’s defence intelligence agency said.
Kremlin spokesman Dmitry Peskov said Russian President Vladimir Putin had been informed, and that work was underway to drive out the "saboteurs", the state-run RIA Novosti news agency reported.
Belgorod governor Vyacheslav Gladkov said on the 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 had been damaged.
Belgorod is 40km from the border with Ukraine and is the largest Russian city in the area.
It has been the site of several attacks since the start of the war, with Ukrainian strikes hit key resources and logistic centres.
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.
The group also released a video showing five heavily 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," the fighter added.
Aleksey Baranovsky, a representative of the Kyiv-based Russian Armed Opposition Political Centre – the political wing of the Freedom of Russia Legion – told CNN that the operation had started Sunday night and fighting was “ongoing.”
He would not specify the number of fighters who had crossed the border into Russia.
Baranovsky said the group wanted to “liberate our motherland from the tyranny of Putin.”
Ukrainian presidential adviser Mykhailo Podolyak tweeted: "Ukraine is watching the events in the Belgorod region of Russia with interest and studying the situation, but has nothing to do with it.
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"As you know, tanks are sold at any Russian military store, and underground guerrilla groups are composed of Russian citizens."
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 said.