President Vladimir Putin has accused Ukraine of carrying out “another major provocation” after defence officials said about 300 Ukrainian troops crossed into Russia’s Kursk region on Tuesday.
Fighting is said to be continuing in the area, with Moscow saying troops backed by 11 tanks and more than 20 armoured fighting vehicles had crossed the border near the town of Sudzha, 10km (six miles) from the frontline.
Officials say thousands of people have fled their homes in the area. Ukraine has yet to comment on Russia’s accusations.
Speaking ahead of a Security Council meeting in Moscow, Mr Putin accused Ukrainian forces of “indiscriminately firing” at civilian buildings and homes.
Fighting reportedly raged in several villages on Russian territory throughout Tuesday, followed by Ukrainian air strikes that killed three civilians and lasted into the night, Russian authorities said.
Moscow said 24 people, including six children, were injured in Ukrainian shelling of the border area.
On Wednesday, the Russian Defense Ministry said it had prevented Ukrainian armed forces from advancing “deep into Russian territory” in the Kursk region and said it had destroyed several Ukrainian drones overnight.
However, several air warnings remain in place in Kursk, where local authorities have urged people to limit their movements and all public events have been cancelled.
Footage posted online – and verified by the BBC – shows fighter jets flying low overhead in the area on Tuesday, with smoke rising from areas on the ground.
The region’s acting governor, Alexei Smirnov, said he had informed Russian President Vladimir Putin about the situation and said it was under control.
Mr Smirnov also said thousands of people had left the hit areas and added that doctors from Moscow and St Petersburg were on their way to help.
Kiev has yet to comment on any reports of events in Kursk.
A colonel in the Ukrainian army, Vladislav Seleznyov, told the popular Nexta TV channel that the offensive was “preventative” as an estimated 75,000 Russian troops continued to mass near the border.
Following Russia’s major cross-border offensive into the northeastern Kharkiv region in May, there are fears Moscow will attempt a similar move into the Sumy region further north.
With Ukraine now occupying several settlements and highways in the opposite direction, those ambitions may have been dashed for now.
But with Ukrainian forces already overstretched and understaffed, some military analysts are questioning the wisdom of such cross-border raids.
This is not the first incursion into Russia by Ukraine-based militants. Several anti-Kremlin Russian groups carried out raids last year, but were repelled.
In March, the forces again entered the Belgorod and Kursk regions, where they clashed with Russian security forces.
