Vladimir Putin is desperately deploying troops to Russia's border amid fears that a significant incursion by Ukrainian forces could see them seize or cut off a key nuclear power plant.
Moscow said it had sent reserves to help repel hundreds of Ukrainian fighters backed by tanks,in a ground incursion that is shaping up to be one the largest into Russian territory during the war.
On Tuesday,the Russian army and security agencies were wrong footed by a breakthrough on the Kursk region frontier,which had been guarded by lightly armed conscripts,several of whom were captured.
Kyiv went on to pummel the region with waves of drone attacks overnight,according to the local governor,with defence systems destroying at least five.
Heavy clashes are expected today as forces from Ukraine - possibly both army and anti-Putin Russian partisans - seek to press deeper into the region.
Pictures shared after yesterday's border breakthrough show damage to buildings
The border town of Sudzha was badly blitzed with most residents evacuating.
Administrative and residential buildings and the infectious diseases department of the city hospital were damaged,reported Izvestia.
Previously,forces describing themselves as voluntary paramilitaries fighting on Ukraine's side inflicted minimal damage in a major incursion into parts of Belgorod and Kursk region this year,but the purpose of the raids remains unclear.
On Tuesday,Ukraine's general staff made no mention of any Ukrainian offensive operation inside Russia.
Throughout the more than two years of war,Ukraine's efforts have largely been focused on fighting back Russian forces who control nearly a fifth of its territory and have made a series of gradual gains in the past six months.
Ukrainian strikes inside Russia's own territory have mostly involved shelling of border regions and drone attacks on targets such as oil refineries and fuel depots.
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