Russia has evacuated more than 76,000 people in the Kursk region as Ukrainian troops push further into its territory.
Vladimir Putin has been forced to call in reserve troops to aid what Moscow is calling a “counterterrorism operation”.
At the same time, a federal-level emergency was declared in Russia over the incursion.
As the attack enters its fifth day, Ukrainian leader Volodymyr Zelensky praised his army’s ability “to surprise” and “achieve results”.
Around 1,000 Ukrainian troops in tanks and armoured vehicles advanced across the Russian border and into the Kursk region in the early hours of Tuesday, with US-based analysts suggesting they had penetrated more than six miles into Russia, breaking through two defensive lines and a stronghold.
The move marks a step change in the conflict and could offer President Zelensky bargaining power with Moscow should he choose to use it. Nonetheless, his aims are as yet unclear.
Overnight, Ukraine claimed to have hit a Russian military airfield in the Lipetsk region – more than 100 miles into Russia – causing a large fire and damaging guided bomb stockpiles at an airfield housing Su-34, Su-35 and MiG-31 warplanes.
In retaliation, the Russian defence ministry said the army had conducted airstrikes against Ukrainian targets, using much-feared thermobaric weapons that create destructive shockwaves when used and also form a vacuum which can suffocate troops.
Ukraine’s navy and military intelligence attacked and damaged a former offshore gas platform used by Russian forces in the Black Sea, the navy spokesperson claimed.
“The occupiers used this location for GPS spoofing to make civilian navigation dangerous. We cannot allow this to happen,” Dmytro Pletenchuk said on Facebook.
He posted a video taken at night showing an explosion on an offshore platform and the ensuing fire.
Mr Pletenchuk said that half a day before the attack, Russian forces had stationed equipment and military personnel on the platform.
“There were no civilians there,” he said.
The president of Moscow’s ally Belarus said his air defences had shot down “several” of about a dozen Ukrainian drones which entered his country’s airspace.
“I do not understand why Ukraine needs this. We need to figure it out. As I said before, we made it clear to them that any provocations will not go unanswered,” Alexander Lukashenko said, according to state news agency Belta.
Belarus’s foreign ministry warned that further airspace violations could mean a diplomatic incident whereby Belarus would call into question the “appropriateness” of Ukraine‘s diplomatic representation in Minsk.
“It was pointed out … that if Ukraine‘s diplomatic representation in Belarus cannot have an effect on preventing such provocations, the Belarusian side will raise the issue of the appropriateness of its continued presence in Minsk,” the ministry said.
Belarus’s defence minister said he would send more troops to its border with Ukraine following the Kursk incursion.
Viktor Khrenin said: “Considering the situation in Ukraine and in the Kursk region of Russia, the commander-in-chief of the armed forces has given orders to reinforce troops in the Gomel and Mozyr tactical areas in order to respond to such provocations.
“Special operations forces, ground troops and rocket forces, including Polonez and Iskander systems, have been tasked with deploying to the designated areas.”
Mr Zelensky has praised his army’s ability “to surprise” and “achieve results” as it battles Russian forces during an audacious cross-border attack into Russian territory.
While Ukrainian officials haven’t spoken directly about the attack, Mykhailo Podolyak, a senior aide to the president, said that “the root cause of any escalation”, including into Kursk, was “unequivocal aggression” on the part of Russia in believing it could invade Ukraine without consequences.
Asked about Ukraine‘s incursion, White House national security spokesperson John Kirby said the United States was “in touch with our Ukrainian counterparts” but that he would not comment further until “those conversations are complete”.
“There’s been no changes in our policy approaches,” Mr Kirby said when asked about US policy on the use of weapons after Ukraine had previously been barred from attacking Russian territory with Western arms.
“They’re using it in an area where we had said before that they could use US weapons for cross-border strikes. The end goal here is to help Ukraine defend itself.”
The US is sending Ukraine an additional $125m (£98m) in weapons to assist in its military operations against Russia, including much-needed air defence capabilities, radars to detect and counter enemy artillery and anti-tank weapons, the White House recently announced.