Russia said on Tuesday that its troops had made major new gains in its western region of Kursk, accelerating their push to drive out Ukrainian forces who have been clinging on to a slice of Russian territory since last August.

Russian forces have begun an assault on the town of Sudzha, the main population center in the Kursk region that was captured by Ukraine last year, a top Russian commander said on Tuesday. The advance puts Moscow on the verge of ending the first invasion of Russian territory since World War II.

Ukraine’s presence in Russia’s Kursk region has deteriorated sharply, with the advance threatening Kyiv’s sole territorial bargaining counter at a crucial time in the war.