Russian President Vladimir Putin has accused Ukraine of a “large-scale provocation” as his officials say they are fighting off cross-border raids for a second day.
Mr Putin accused Ukrainian forces of “indiscriminate shelling of civilian buildings, residential houses, ambulances with different types of weapons” in an incursion into Russia‘s south-western Kursk region.
Ukrainian officials have not commented on Russia’s claims.
If confirmed, it would be among the largest incursions into Russia since the war began.
Russia’s defence ministry said on Wednesday its military and border guard troops “continued to destroy Ukrainian military units in the areas alongside the border in the Kursk region”.
It said it had destroyed 50 armoured vehicles, including seven tanks, eight armoured personnel carriers, three infantry fighting vehicles and 31 armoured combat vehicles in the area.
The ministry said that Russian forces backed by artillery and warplanes “didn’t allow the enemy to advance deeper” into Russian territory.
A day earlier, it said up to 300 troops, supported by 11 tanks and more than 20 armoured combat vehicles, had crossed into Russia.
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Local people were urged to donate blood by the acting governor of the Kursk region, who said on Telegram that blood banks were stocking up because of the fighting.
Alexei Smirnov said: “In the last 24 hours, our region has been heroically resisting attacks” by Ukrainian fighters.
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If confirmed, the reported advance could be an attempt to draw Russian reserves to the area, weakening Moscow’s offensive operations in several sectors of Ukraine’s eastern Donetsk region, where Russian forces have ramped up attacks.
But it could also risk stretching outmanned Ukrainian troops further along the front line, which is more than 620 miles (1,000 km) long.
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After its 2023 counter-offensive stalled, Kyiv’s forces have struggled to contain Russian advances this year.
Kyiv is concerned that US support could drop off if Donald Trump, who has said he would end the war, wins November’s presidential election.
Russian military bloggers reported intense battles in the region with some suggesting that Ukraine had opened a new front.
Yuri Podolyaka, an influential Ukrainian-born pro-Russian military blogger, said: “The fighting will be fierce. It’s definitely not going to end quickly.
“Even if the enemy fails to break through (and no one will give guarantees for now) there will be artillery and drone attacks. And in large numbers.”
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Other bloggers said Ukrainian soldiers were in Kursk and had seized three settlements in the region and continued to fight their way deeper into it.
Mr Smirnov said there had been rocket and drone attacks overnight, and told civilians to take cover away from windows.
He said a Ukrainian attack drone had hit an ambulance outside the border town of Sudzha, killing two people and another report claimed Ukrainian shelling had hit a cathedral.
Both Kyiv and Moscow say their attacks do not target civilians but thousands have died since Russia invaded its neighbour in 2022.
Russian forces have swiftly repelled previous cross-border incursions.
Forces describing themselves as voluntary paramilitaries fighting on Ukraine’s side penetrated parts of Belgorod and the Kursk region this year, triggering a major push by Russian troops to carve out a buffer zone in Ukraine’s northeast.