Ukraine has chalked up a string of victories more than a week since blindsiding Russia with a lightning cross-border assault, but the risks are piling up as its troops make plans to hold territory and Russia recovers its footing.

Ukraine poured thousands of troops into the western Russian region of Kursk last week, pulling down Russian flags in towns seized by its soldiers and wresting the war initiative from Moscow for the first time in months.

On Wednesday, officials in Kyiv said Ukraine would use seized Russian territory as a "buffer zone" to shield its north from Russian strikes. Oleksandr Syrskyi, head of the Ukrainian armed forces, said on Thursday that Kyiv had set up a military commandant's office in the occupied part of Kursk, suggesting ambitions to dig in.