Russian troops are fighting Ukrainian forces in the Kursk region on the third day of one of the largest cross-border incursions of the war, the Russian Defense Ministry said Thursday.

A ministry statement said the Russian military and border guards have blocked Ukrainian forces from advancing deeper into Kursk region in southwest Russia and that the army is attacking Ukrainian combatants who are trying to advance on the area from Ukraine’s Sumy region.

“Attempts by individual units to break through deep into the territory in the Kursk direction are being suppressed,” the ministry said.

Ukrainian troops had advanced as much as 15 kilometers (9 miles) into Russian territory, according to the Institute for the Study of War, a Washington-based think tank, but that information hasn’t been confirmed. Ukrainian officials haven’t commented on the scope of the operation around the town of Sudzha. Analysts say all available open source information suggests Ukrainian forces are advancing.The enemy has not advanced a single meter, on the contrary, it is retreating. The enemy’s equipment and combat forces are being actively destroyed. We hope that in the near future … the enemy will be stopped,” the Kursk region’s acting deputy governor Andrei Belostotsky said Thursday, according to state news agency RIA-Novosti.