Russia's military has said it is "continuing to repel" a Ukrainian cross-border incursion into the western Kursk region - a surprise attack now in its fourth day.
The Russian defence ministry said Ukraine lost more than 280 military personnel in the past 24 hours - a claim that has not been independently verified.
Reports suggest that Ukrainian troops are operating more than 10km (six miles) inside Russia - the deepest cross-border advance by Kyiv since Moscow launched a full-scale invasion in February 2022.
Ukraine has not openly admitted the incursion, but President Volodymyr Zelensky said on Thursday that Moscow must "feel" the consequences for its invasion.
Meanwhile, 13 people have been killed and at least 43 injured in a Russian strike on a shopping centre in the Ukrainian town of Kostiantynivka, close to the frontline in Donetsk region, Ukrainian emergency officials say.
Residential buildings, shops and more than a dozen cars were also damaged in the attack.
News of the strike came hours after Ukraine's military said it had hit a military airfield deep inside Russia overnight, destroying a warehouse containing hundreds of glide bombs.
The targeting of the Lipetsk air base, more than 350km (217 miles) from Ukraine's border, is the kind of operation Kyiv has been wanting to do for some time.
These are the very tools that Russia has continually terrorised Ukrainian towns, cities and military positions with for most of its invasion.
The military's statement also said the airfield was known for housing Russia's Su-34, Su-35 and MiG-31 war planes.
Lipetsk's regional authorities said a state of emergency was now in place in the area, confirming the detonations at an "energy infrastructure facility". Residents of four nearby villages were being evacuated.
In a statement on Tuesday, the Russian defence ministry said its forces were repelling "an attempt by the Ukrainian armed forces to invade the territory of the Russian Federation".
It said Russia was using aviation and artillery, managing to suppress "raid attempts by enemy units".
But a video checked by OceanNewsUK shows a different picture, with a 15-vehicle Russian convoy damaged, burned and abandoned on a road through the town of Oktyabr'skoe, roughly 38km (24 miles) from the border on the Russian side.
The early morning footage also shows Russian soldiers, some injured, possibly dead among the vehicles.
A "federal state of emergency" has been declared in the Kursk region - a move that underlines how grave the current situation is.
Russia said that up to 1,000 Ukrainian troops, supported by tanks and armoured vehicles, entered the Kursk region on Tuesday morning.
Despite the deployment of reserve troops and orders to evacuate, Russia has been unable to slow the momentum of this Ukrainian advance.
This is more than the probing attacks we have seen in the past. Hundreds of soldiers with armoured vehicles are thought to have made it up to 10km into Russian territory.
It is a committed assault which has shocked Russia’s military and the Kremlin. For the last 18 months it has been Moscow dictating the dynamics of this war.
Now it is having to both contain this attack as well as domestic criticisms for not preventing it in the first place.
Despite long-time Western worries of an escalation, the consensus among Ukraine’s allies is that this operation falls within its right to defend itself.
While he is yet to directly reference the assault, President Zelensky said in a video address late on Thursday: “Russia has brought the war to our land and should feel what it has done.”
But with his Ukrainian forces still outnumbered by the Russians on the battlefield, the line between masterstroke and miscalculation is a fine one.
