Russia said on Friday its forces were continuing to create a "security strip" in border areas of Ukraine's Sumy region after driving Ukrainian troops out of the Kursk region, just across the border in western Russia.
Ukraine says its forces still have a foothold in Kursk, where it staged an incursion in August to try to distract Russian forces and win leverage in any future peace talks, but that it is concerned about a possible Russian advance into Sumy.
Russian President Vladimir Putin said in March that Russia should look to create a buffer zone in the Sumy region to guard against any future potential Ukrainian incursions.
"Units of the North group of forces have completed the rout of Ukrainian Armed Forces formations in the Kursk region," the Russian defense ministry said in a statement.