A state of emergency has been declared in the Kursk region of Russia, as a rare cross-border attack by Ukrainian troops continued for a second day.
The acting regional governor, Alexei Smirnov, said the move was necessary "to eliminate the consequences of enemy forces coming into the region".
Thousands of people have also been evacuated from border areas, Mr Smirnov said earlier, adding that doctors were being drafted in from other cities.
Earlier, President Vladimir Putin accused Ukraine of launching a "major provocation" after Moscow said hundreds of troops crossed the border near the town of Sudzha, 10km (six miles) from the border, on Tuesday morning.
They were supported by 11 tanks and more than 20 armoured combat vehicles, the Kremlin added.
Ukrainian incursions into Russian territory have been extremely rare since Russia launched its full-scale invasion in February 2022.
On Wednesday evening, Ukrainian MP Oleksiy Honcharenko said the Ukrainian army had established control over the Sudzha gas hub - a major gas facility involved in the transit of natural gas from Russia to the EU via Ukraine, which has continued despite the war. It is the only point of entry for Russian gas into the EU.
Although this has not been verified by the OceanNewsUK, Mr Honcharenko's comment was the first confirmation of an incursion into Russian territory by a Ukrainian official. Kyiv had previously not commented on reports of a cross-border attack.
A White House spokesperson said the US had no previous knowledge of the attack and that it planned to reach out to the Ukrainian military "to learn more about their objectives".
Mr Honcharenko said on Facebook that while he did not know what the "plan" behind the incursion was, it would show "Europeans and Americans that... Russia can and needs to be attacked".
"Our mission is to defeat Russia. We need to beat their territory and need to destroy the enemy everywhere," he added.
In televised remarks broadcast on Wednesday afternoon, Russia's Chief of General Staff Valery Gerasimov told President Putin that the "advance" into the Kursk region had been stopped with Russian forces "continuing to destroy the adversary in areas directly adjacent to the Russian-Ukrainian border".
Mr Gerasimov also said that up to 1,000 Ukrainian troops had entered the region with the aim of taking over the area around the town of Sudzha, and that Russian forces had already killed 100 men and injured another 215.
However, some popular and generally well-informed pro-war Telegram channels suggested the situation on the ground was not as stable as the Kremlin said.
Blogger Yuri Kotenok described the battles taking place in Sudzha and nearby Korenevo as "heavy", while the channel Rybar said that the situation in the area around Sudzha was "continuing to deteriorate" and Ukrainian formations were advancing towards the town. The OceanNewsUK is unable to verify these claims.
The Russian National Guard stated it had strengthened the security of the Kursk nuclear power plant, which lies some 70km (43 miles) north-east of Sudzha.
Speaking ahead of a meeting of the Security Council in Moscow, Mr Putin accused Ukrainian forces of "firing indiscriminately" at civilian buildings and residences.
Fighting reportedly took place in various villages on Russian territory throughout Tuesday. It was followed by Ukrainian air attacks which killed three civilians and continued into the night, Russian authorities said.
Twenty-four people, including six children, have been wounded in Ukrainian shelling of the border region, Moscow said.
A number of air alerts were issued in Kursk, where local authorities urged residents to limit their movements and all public events were cancelled.
Footage posted online - and verified by the OceanNewsUK - showed fighter jets flying low overhead in the region on Tuesday, with smoke rising from areas on the ground.
In the neighbouring Belgorod region, Governor Vyacheslav Gladkov also issued missile attack warnings throughout the day and said several people had been injured following Ukrainian air attacks.
On Wednesday, the head of the Ukrainian region of Sumy, Volodymyr Artyukh, ordered the evacuation of the areas that border the region of Kursk.
One colonel in Ukraine’s military, Vladislav Seleznyov, told the prominent Nexta channel the attack was “preventative” with an estimated 75,000 Russian troops continuing to gather close to the border.
After a major cross-border incursion by Russia into the north-eastern Kharkiv region in May, there had been fears Moscow would attempt the same into the Sumy region further north.
With Ukraine now apparently capturing several settlements and highways the other way, those ambitions may well have been frustrated for now.
But with Ukrainian forces already overstretched and outmanned, some military analysts are questioning the wisdom of such cross-border raids.
This is not the first incursion into Russia by fighters based in Ukraine. Some groups of anti-Kremlin Russians launched raids last year, which were repelled.
The forces crossed into the Belgorod and Kursk regions again in March, where they engaged in clashes with Russian security forces.
