Russia on Friday said it was deploying more troops and munitions to a frontier region where Ukraine has mounted a major ground offensive, while Russia struck a supermarket in east Ukraine, killing at least 10 people.
Kyiv's troops have been driving into Russia's western Kursk region since Tuesday in a surprise offensive that appears to be the most significant attack on Russian soil since Moscow invaded in February 2022.
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Russia's defence ministry said it was sending columns of military hardware, including rocket launchers, artillery, tanks and heavy trucks to reinforce its defences in the region, state media reported Friday.
The Russian army said around 1,000 Ukrainian troops and more than two dozen armoured vehicles and tanks were involved in the initial attack, although it has since claimed to have destroyed many more pieces of hardware.
Kyiv has not officially taken responsibility for the attack, but Ukrainian President Volodymyr Zelensky has said Russia needed to "feel" the consequences of its invasion.
Both sides also stepped up aerial attacks behind the frontlines on Friday.
A Russian artillery strike on a supermarket in the east Ukrainian town of Kostyantynivka during the middle of the day killed at least 10 people and wounded 35, Ukrainian Interior Minister Igor Klymenko said in a post on Telegram.
The town is about 13 kilometres (eight miles) from the nearest Russian positions and faces almost daily strikes.
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"Russia will be held accountable for this terror," Zelensky said in a post on Telegram.
AFP journalists on the scene saw dozens of people fleeing as police officers warned of a potential second strike.
Plumes of black smoke rose from the building and police cordoned off nearby streets. Drones and an uptick in artillery fire in the area were also audible.
- 'Russia brought the war' -
Russian troops have been slowly advancing for months in the Donetsk region where Kostyantynivka is located.
Ukraine's surprise offensive into the Kursk region has appeared to catch Russia off guard, with some analysts saying it may be an attempt to force Moscow to divert resources and relieve pressure on other parts of the sprawling front line.
Influential Russian military bloggers have blasted army leaders for failing to spot or quash the incursion.
Senior Kyiv officials have stayed largely tight-lipped, although Zelensky on Thursday appeared to justify the attack.
"Russia brought the war to our land and should feel what it has done," he said, without directly referring to the ongoing Ukrainian incursion.
"Everyone can see that the Ukraine army knows how to surprise and knows how to achieve results," he said in another apparent reference to the incursion.
Moscow has also not presented detailed information on the extent of the Ukrainian advance.
The defence ministry said on Friday its troops "continue to repel an attempted incursion by the Ukrainian armed forces" and that it had struck Ukrainian positions on the western edge of Sudzha, a town around eight kilometres (five miles) from the border.
- 35 kilometres -
Based on geolocated videos and photos, the independent US-based Institute for the Study of War (ISW) said Ukrainian units had pierced much further into Russian territory in a "rapid advance".
"Ukrainian forces are reportedly present in areas as far as 35 kilometres from the international border," the ISW said in its daily campaign assessment.
It cautioned, however, that Ukrainian forces "most certainly do not control" that entire area.
Russian President Vladimir Putin has called the incursion a "large-scale provocation" by Kyiv, and Russia's top general has vowed to crush it.
Moscow has already rushed reserves to the region and used drones, artillery and aviation to try to quash the attack.
The health ministry said 66 civilians were wounded, including nine children, in the first three days.
On the first day of the assault, Kursk regional governor Alexei Smirnov said five civilians were killed.
Thousands have been evacuated from the region, with Russia putting on an extra train to Moscow from the regional capital, Kursk, for those looking to leave.
Ukraine on Friday expanded its own evacuation zone in the Sumy region, just across the border from Kursk.
"About 20,000 people need to be evacuated" from 28 settlements, Ukraine's national police force said.
Ukraine also said on Friday it had carried out a major air strike on a Russian military base in the Lipetsk region, around 280 kilometres (175 miles) from the Russia-Ukraine border.
It said it had struck "warehouses containing guided aerial bombs and a number of other facilities."
Local Russian officials had earlier reported a "massive" Ukrainian drone attack on the region and state media reported a fire had broken out at the airbase.
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