Russia on Friday deployed more troops and munitions to a border region where Ukraine has mounted a major ground offensive, the defence ministry said.
Ukraine said meanwhile that a Russian strike on a supermarket in the east of its territory had killed 14 people.
Ukrainian troops launched a surprise offensive into Russia's western Kursk region on Tuesday, in the most significant attack across the border since Moscow invaded in February 2022.
Russia's defence ministry said it was sending rocket launchers, artillery, tanks and heavy trucks to reinforce its defences in the region, state media reported.
Around 1,000 Ukrainian troops and more than two dozen armoured vehicles and tanks were involved in the initial attack, said Moscow, though it later claimed to have destroyed many more pieces of equipment.
While Ukraine has not officially confirmed the offensive, President Volodymyr Zelensky said Thursday that Russia needed to "feel" the consequences of its invasion.
On Friday, both sides stepped up aerial attacks behind the frontlines.
A Russian missile strike on a supermarket and post office in the east Ukrainian town of Kostyantynivka killed at least 14 people and wounded 43, Ukraine's general prosecutor said.
The town is about 13 kilometres (eight miles) from the nearest Russian positions.
"Russia will be held accountable for this terror," Zelensky said in a post on Telegram.
'The war has come to us'
Ukraine's offensive into the Kursk region appeared to catch Russia off guard.
Influential Russian military bloggers have blasted army leaders for failing to spot or quash the incursion.
Senior Ukrainian officials have not commented although Zelensky alluded to the attack on Thursday.
"Everyone can see that the Ukraine army knows how to surprise and knows how to achieve results," he said.
Moscow has not presented detailed information on the extent of the advance.
It said Friday it had struck Ukrainian positions on the western edge of Sudzha, a town around eight kilometres (five miles) from the border that appeared to be the focus of Kyiv's offensive.
Several Russian media shared a video purporting to show Sudzha residents appealing to President Vladimir Putin for help, warning that many were unable to evacuate.
"In a few hours our town was turned into ruins ... Our relatives are left behind, we can't call them, and there is no communication. Please help us get our land back," one resident said in the video.
Thousands have been evacuated from the border region, with Russia putting on an extra train to Moscow from the regional capital, Kursk.
At a train station in Moscow, AFP journalists saw families disembarking with children.
"The war has come to us, so all the relatives have gone to Moscow," AFP heard a woman with her young daughter saying at the station.
'Rapid advance'
"Ukrainian forces are reportedly present in areas as far as 35 kilometres from the international border," said the independent US-based Institute for the Study of War (ISW) in its daily assessment drawing on geolocated videos and photos.
But those troops "most certainly do not control" that entire area, it added.
Putin has called the incursion a "large-scale provocation" by Kyiv, and Russia's top general has vowed to crush it.
On the first day of the assault, Kursk regional governor Alexei Smirnov said five civilians had been killed.
The health ministry said Thursday that 66 people had been wounded. Twelve were hospitalised in a serious condition, it said Friday.
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 police force said.
Ukraine also said 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".
The International Atomic Energy Agency issued a statement warning both sides of the dangers as the fighting neared the Kursk nuclear power plant.
"I would like to appeal to all sides to exercise maximum restraint in order to avoid a nuclear accident with the potential for serious radiological consequences," said IAEA chief Rafael Grossi.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
