Russia ordered more evacuations of border regions as it battled an unprecedented Ukrainian incursion across two fronts on Monday.
Ukraine last week sent troops into Russia's border region of Kursk, in the largest cross-border operation by Kyiv since Moscow launched its offensive more than two years ago.
The assault, which has sent tens of thousands of people fleeing, marked the most significant attack by a foreign army on Russian territory since World War II.
A top Ukrainian official told AFP that the operation was aimed at stretching Moscow troops and destabilizing the country after months of slow Russian advances across the frontline.
The assault appeared to catch the Kremlin off guard, with Moscow's army rushing in reserve troops, tanks, aviation, artillery and drones in a bid to quash it.
Despite the efforts, the army Sunday conceded that Ukraine had been able to penetrate its territory by up to 30 kilometers (20 miles) in places.
In a daily briefing on the situation in the western Kursk region, the Defense Ministry said it had "foiled attempts" by Ukraine's forces to "break through deep into Russian territory" using armored vehicles.
But it said some of those forces were near the villages of Tolpino and Obshchy Kolodez, some 25 kilometers and 30 kilometers from the Russia-Ukraine border.
A Ukrainian security official told AFP on condition of anonymity that "the aim is to stretch the positions of the enemy, to inflict maximum losses and to destabilize the situation in Russia as they are unable to protect their own border."
The Ukrainian official also said Russian claims that Kyiv had deployed 1,000 troops were a serious underestimate.
"It is a lot more," he said. "Thousands."
On Monday, Russia ordered fresh evacuations in the region, moving people from the neighboring region of Belgorod.
"The enemy is active on the border of the Krasnoyaruzhsky district," Vyacheslav Gladkov said on Telegram.
"For the health and security of our population, we're beginning to move people who live in Krasnoyaruzhsky to safer places."
Russia's Defense Ministry said on Monday that its air defense systems had destroyed 18 Ukrainian drones – including 11 over the Kursk region.
Helicopters 'over your head'
On Sunday, each country blamed the other for a fire at the Zaporizhzhia nuclear plant in southern Ukraine occupied by Russian forces, though both sides – and the U.N.'s nuclear watchdog – said there was no sign of a nuclear leak.
"No impact has been reported for nuclear safety," said a statement from the International Atomic Energy Agency, which has experts at the site. Both Kyiv and Moscow said there had been no rise in radiation levels.
In a later statement, the IAEA said it had requested that its team get "immediate access to the cooling tower to assess the damage."
A Moscow-installed official, Vladimir Rogov, said the blaze has been "completely extinguished" in a Telegram post on Monday.
Russia invaded Ukraine in February 2022 and has waged an unrelenting campaign, occupying swathes of the east and south and subjecting Ukrainian cities to daily missile and drone attacks.
After recapturing large areas in 2022, Kyiv has largely been on the back foot, struggling with manpower and arms supplies.
Russia said Saturday that more than 76,000 civilians had been evacuated from border areas, with more leaving Sunday.
Russia's rail operator has put on emergency trains from Kursk to Moscow, around 450 kilometers away, for those looking to flee.
"It's scary to have helicopters flying over your head all the time," said Marina, refusing to give her surname, who arrived by train in the Russian capital Sunday. "When it was possible to leave, I left."
Kursk regional governor Alexei Smirnov conceded on Sunday that the situation was "difficult."
Across the border in Ukraine's Sumy region, from where Ukraine launched the incursion, AFP journalists Sunday saw dozens of armored vehicles daubed with a white triangle – the insignia apparently being used to identify Ukrainian military hardware deployed in the attack.
'Taste' of war
At an evacuation center in the regional capital of Sumy, 70-year-old retired metal worker Mykola, who had fled his village of Khotyn some 10 kilometers from the Russian border, nevertheless welcomed Ukraine's push into Russia.
"Let's let them find out what it's like," he told AFP. "They don't understand what war is. Let them have a taste of it."
Analysts think Kyiv may have launched the assault to try to relieve pressure on its troops in other parts of the sprawling front line.
But the Ukrainian official said: "Their pressure in the east continues, they are not pulling back troops from the area," even if "the intensity of Russian attacks has gone down a little bit."
The Ukrainian official said he expected Russia would "in the end" manage to stop the incursion.
Ukraine was bracing for retaliation with a large-scale missile attack, including "on decision-making centers" in Ukraine, the official added.