Russia said on Monday its forces had made important gains in eastern Ukraine while continuing to fend off a new Ukrainian offensive inside the Kursk region of western Russia, where a second day of fierce fighting was under way.

The surprise attack seeks to reverse Ukraine’s decreasing hold of the region. Since Kyiv’s forces gained partial control of Kursk following a daring cross-border assault last August, Russian troops, aided by North Korean soldiers, have halved the territory held by Ukraine, albeit while suffering thousands of casualties.

Russian forces continue to make costly but consistent gains in the Donetsk region in eastern Ukraine. They are steadily grinding down a pocket of resistance around the town of Kurakhove and are also fighting to envelop the larger nearby city of Pokrovsk, Ukrainian soldiers and officials said.