Ukraine on Wednesday pressed its surprise offensive in Russian territory and bombarded the neighbouring border region of Belgorod, where the governor has declared a state of emergency.
Ukrainian forces entered Kursk region last Tuesday and have taken dozens of settlements in the biggest attack by a foreign army on Russian soil since World War II.
President Volodymyr Zelensky on Tuesday evening posted footage of a video call with his military chief Oleksandr Syrsky, who said that "as of today, our troops have advanced in some areas by one to three kilometres."
Over the last day, Syrsky added, "control over 40 square kilometres of territory has been taken" and "74 settlements are under our control".
The head of neighbouring Belgorod declared a state of emergency on Wednesday, saying the situation was "extremely difficult" in the Russian border region under Ukrainian bombardment.
"The situation in our Belgorod region remains extremely difficult and tense due to shellings from the Ukrainian Armed Forces. Houses are destroyed, civilians died and were injured," governor Vyacheslav Gladkov said on Telegram.
Belgorod, which borders Ukraine's Kharkiv region, had also come under drone attack, he said.
After a week of the Ukrainian advance, Syrsky said Monday that his troops controlled about 1,000 square kilometres (386 square miles) of Russian territory.
Kursk regional governor Alexei Smirnov said Monday that 28 settlements had been captured and Ukrainian forces had penetrated at least 12 kilometres deep.
An AFP analysis of data provided by the Institute for the Study of War (ISW) indicated that Ukrainian troops had advanced over an area of at least 800 square kilometres of Russian territory as of Monday.
Joe Biden on Tuesday said the incursion was giving Russia's President Vladimir Putin a "real dilemma," in the US leader's first comments on Kyiv's surprise attack.
Russia's defence ministry said Tuesday its troops "continue to repel" Ukrainian assaults and had "foiled" attacks by "mobile groups in armoured vehicles".
Ukraine said it would not hold on to Russian territory it had captured and offered to stop raids if Moscow agreed a "just peace".
Foreign ministry spokesman Georgiy Tykhy said Kyiv was not interested in "taking over" Russian territory and defended Ukraine's actions as "absolutely legitimate".
"The sooner Russia agrees to restore a just peace... the sooner the raids by the Ukrainian defence forces into Russia will stop," he told reporters.
More than 120,000 Russians have fled their homes in border areas of Kursk region.
Ukraine said it was imposing movement restrictions in the Sumy region along the border due to an "increase in the intensity of hostilities" and "sabotage" activities.
Since launching its invasion in February 2022, Russia has captured territory in southern and eastern Ukraine and subjected Ukrainian cities to missile and drone barrages.
Ukraine has found itself outmanned and outgunned and has struggled after some early successes in pushing Russian forces back in 2022.
The current offensive into Kursk region, which caught Russia off guard, is by far the biggest cross-border action since the invasion.
On the Ukraine side of a border crossing into Kursk, AFP reporters saw toppled concrete fortifications and caved-in remains of security and customs buildings revealing the intensity of the fighting that swept through the area.
On the road, around 10 blindfolded and bound men in Russian military fatigues were being driven in a military vehicle away from the border crossing in the direction of the city of Sumy.
"They didn't protect the border," a Ukrainian serviceman who took part in the offensive and identified himself as Ruzhyk told AFP in Sumy region.
"They only had anti-personnel mines scattered around trees at the side of the road and a few mines that they managed to quickly throw along the highways," he said.
Another serviceman said that his unit of military engineers went in to clear the mines before Ukrainian tanks entered.
"But when our guys came in, the Russians ran away. They didn't have time to press the detonate buttons."
A 27-year-old squad leader, who identified himself as Faraon, was sparing but direct in his description of battles in Kursk.
"I saw a lot of death in the first few days. It was terrifying at first but then we got used to it," he told AFP.
"There have been many deaths," he repeated, standing next to a forest road leading to the frontier, without elaborating.
Putin has vowed to "dislodge" Ukrainian troops. He told a televised meeting with officials on Monday that "one of the obvious goals of the enemy is to sow discord" and "destroy the unity and cohesion of Russian society".
Putin also said Ukraine wanted to "improve its negotiating position" for any future talks with Moscow.
A Ukrainian security official told AFP, on condition of anonymity, at the weekend, that Ukraine sought to "stretch the positions of the enemy, to inflict maximum losses and to destabilise the situation in Russia as they are unable to protect their own border".
The Ukrainian official said thousands of Ukrainian troops were involved in the operation.
