Vladimir Putin’s security forces came under attack in at least three locations within Russia by anti-regime groups, British defence chiefs said on Tuesday.
The clashes close to the border with Ukraine involved “small-arms fire and drone attacks,” they added, and led to some villages being evacuated.
Russia was now facing an “increasingly serious multi-domain security threat” in its border regions, they stressed.
In its latest intelligence update, the Ministry of Defence in London said: “Between 19 and 22 May 2023, Russian security forces highly likely clashed with partisans in at least three locations within Russia’s Belgorod Oblast, near the Ukrainian border.
“The identity of the partisans remains unconfirmed, but Russian anti-regime groups claimed responsibility.
The most serious incident took place near the town of Grayvoran. As well as small-arms fire fights, there was an uptick in drone or indirect fire attacks near the incidents.
Russia has evacuated several villages and has deployed extra security forces to the area.”
The briefing added: “Russia is facing an increasingly serious multi-domain security threat in its border regions, with losses of combat aircraft, improvised explosive device attacks on rail lines, and now direct partisan action.
“Russia will almost certainly use these incidents to support the official narrative that it is the victim in the war.”
Russian forces pressed on with a "counter-terrorism operation" in a border district on Tuesday, a regional governor said, a day after what appeared to be one of the biggest cross-border incursions from Ukraine since the war began 15 months ago.
Russia said on Monday it was battling an incursion by Ukrainian saboteurs who crossed into the Belgorod region. A Ukrainian presidential adviser said on Twitter the Kyiv government was watching the situation but "has nothing to do with it".
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, both consisting of Russian citizens, were responsible for the attack.
The Russian regional governor, Vyacheslav Gladkov, said on the Telegram messaging service a "counter-terrorism operation" was still going on.
"The cleaning of the territory by the Ministry of Defence together with law enforcement agencies continues," Gladkov said.
On Monday, Gladkov said at least eight people had been wounded, several buildings damaged and many residents had left. He said he had restricted movements and communications.
Gladkov said two buildings were attacked by drones overnight and people could not return to their homes.
"I now appeal to the residents of the Graivoron district, who ... temporarily left their homes, it is not possible to return yet," he said.
Gladkov said one woman died during the evacuation on Monday and there were reports of two people wounded. The "number one" task on Tuesday was to reach them, he said.
The Russia Volunteer Corps published video footage late on Monday showing 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.
Kremlin spokesman Dmitry Peskov said Mr Putin had been informed and that work was under way to drive out the "saboteurs", the state-run RIA Novosti news agency reported.
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 into Russia. The "Open Belgorod" Telegram channel said power and water had been cut off to several villages.
The Liberty of Russia Legion 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.
Russia launched a full-scale invasion of Ukraine in February last year.
The Kremlin said the incursion by a "Ukrainian sabotage and reconnaissance group" was aimed at distracting attention from the eastern Ukrainian town of Bakhmut, which Russian forces say they captured in its entirety after more than nine months of fighting.
Russia says capturing Bakhmut opens the way to further advances in the eastern industrial region known as the Donbas. Ukraine says its advance on the Russian forces' flanks is more meaningful than its withdrawal from Bakhmut itself, and Russia would have to weaken its lines elsewhere to send reinforcements to hold the shattered city.
There were more than 30 clashes in the main sectors of the frontline with the epicentre of fighting remaining Bakhmut and Maryinka further south, the Ukrainian military said early on Tuesday.
"Battles for the city of Bakhmut continue,” it said, with a more widespread counter offensive expected to be launched within weeks.
