Around 10,000 Russian soldiers are fighting in Russia’s Kursk region, about 90 square kilometers (35 square miles) of which is controlled by Ukraine, Ukraine’s top military commander said.

“We control about 90 square kilometers of territory in the Hlushkov district of the Kursk region of the Russian Federation, and these are our preemptive actions in response to a possible enemy attack,” Oleksandr Syrskyi said without elaborating, in remarks released by his office for publication on Sunday.

The Ukrainian military said the activity in this area prevented Russia from sending a significant number of its forces to Ukraine’s eastern region of Donetsk, where some of the heaviest fighting has taken place in the more than three-year-old full-scale invasion.