Russian commanders "face an acute dilemma" over whether to strengthen border defences, or reinforce lines in occupied Ukraine, according to the latest UK defence intelligence.
The Ministry of Defence (MOD) said Russia's Belgorod region has been attacked by partisan groups for the second time in 10 days.
The MOD update said uncrewed aerial vehicles (drones) struck Belgorod city, which is 35km within Russia, whilst Ukrainian shelling prompted the evacuation of civilians from the border town of Shebikino.
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According to UK defence intelligence, Russian military and interior forces have "likely seen quicker success in containing this raid" than the previous attack.
It said a Russian MOD press release said Moscow had deployed "a full range of military firepower on its own territory", including attack helicopters and TOS-1A heavy thermobaric rocket launchers.
UK Foreign Secretary James Cleverly has said Ukraine has the right to "project force" beyond its own borders.
His comments came after Vladimir Putin accused Ukraine of "terrorist activity" after Moscow was hit by a drone attack.
Speaking after the drone strike, Mr Cleverly, who was in Estonia visiting RAF personnel deployed to the country, said: "Ukraine does have the legitimate right to defend itself."