Russia's army was fighting a major Ukrainian border incursion for the third day running, Moscow said Thursday, with independent analysts saying Kyiv's troops had advanced up to 10 kilometres (six miles) into Russia.
Pro-Kyiv forces stormed into Russia's southwestern Kursk region on Tuesday morning, deploying around 1,000 troops and more than two dozen armoured vehicles and tanks, according to the Russian army.
It appears to be the most significant of several cross-border attacks mounted by Ukraine since Russia launched its full-scale military offensive in February 2022.
Russia's defence ministry said Thursday its troops were "continuing to destroy" armed Ukrainian units and were using air strikes, rocket and artillery fire to try to push them back.
It said it had rushed in reserves and was "thwarting attempts to break through" deeper into the Kursk region.
The independent US-based Institute for the Study of War (ISW) said Ukraine had made significant territorial gains in the first two days of the incursion.
"Ukrainian forces have made confirmed advances up to 10 kilometres (six miles) into Russia's Kursk Oblast amid continued mechanised offensive operations on Russian territory," it said in its latest update.
"The current confirmed extent and location of Ukrainian advances in Kursk Oblast indicate that Ukrainian forces have penetrated at least two Russian defensive lines and a stronghold," it added.
Kyiv has not officially taken responsibility for the operation, but an aide to Ukrainian President Volodymyr Zelensky said Moscow was to blame for the incursion.
"The root cause of any escalation, shelling, military actions, forced evacuations and destruction of normal life forms -- including within the Russian Federation's own territories like Kursk and Belgorod regions -- is solely Russia's unequivocal aggression," presidential aide Mykhailo Podolyak said Thursday on social media.
Without mentioning the Kursk attacks, Zelensky himself said Wednesday: "The more pressure we put on Russia ... the closer we will get to peace. A just peace through just force."
Russian President Vladimir Putin has called it a "large-scale provocation" by Kyiv and Russia's top general vowed on Wednesday to crush the incursion.
There have been few detailed updates from Russian officials on the situation on the ground.
The surprise attack appeared to catch Russia's army off guard, triggering criticism of its top brass by the country's influential military bloggers.
The Ukrainian advance has centred on the logistical hub of Sudzha, a town of around 5,000 inhabitants located eight kilometres (five miles) from the Ukrainian border.
Russian military bloggers, who have links to the army, also reported Thursday that Kyiv had made significant advances.
"The situation is complicated and continues to worsen," blogger Yury Podolyaka said in a post on Telegram.
"Sudzha has been completely lost," he said.
The Dva Mayora Telegram channel said Ukraine was "entrenching, indicating that the fighting is likely to be long-term."
Many reported Ukrainian troops had advanced towards the town of Korenevo, more than 25 kilometres from the Ukrainian border.
AFP could not verify those reports.
Russian energy giant Gazprom said on Thursday it would continue shipping gas through the Sudzha metering station, the last transit point for Russian gas heading to Europe via Ukraine.
Moscow also said its forces had destroyed a US-made Bradley Fighting Vehicle that Ukraine was using in the incursion.
The defence ministry had initially blamed a pro-Ukrainian "sabotage group" for carrying out the raid, but it has since said Ukraine's army is carrying out the raid.
Pro-Kyiv militia, including those made up of Russian citizens fighting against Moscow, have staged several previous cross-border raids.
But the latest incursion into the Kursk region appears to be the most successful and organised attempt by Ukraine to penetrate Russian territory.
Kyiv's objectives remain unclear, with some analysts suggesting it is an attempt to stretch Russia's army, forcing them to move units away from the eastern Donetsk region where they have been advancing.
The White House said Wednesday it was contacting Ukraine to learn more about the "objectives" of the incursion.
The governor of the Kursk region introduced an "emergency situation" in the region on Wednesday evening, a move that gives authorities the power to restrict movement in a bid to bring the situation under control.
Several thousand have been evacuated on both sides of the border.
