Five months after Ukrainian forces swept across the border in the first ground invasion of Russia since World War II, the two armies are engaged in some of the most furious clashes of the war there, fighting over land and leverage in the conflict.

The intensity of the battles recalls some of the worst sieges of eastern Ukraine over the past three years, including in towns like Bakhmut and Avdiivka, names that now evoke memories of mass slaughter for soldiers on both sides.

The fighting, in the Kursk region of Russia, has taken on a layer of significance for the territory’s potential to play a role in any ceasefire negotiations. Facing the prospect of an unpredictable new U.S. president — who has vowed to end the war swiftly, without clarifying the terms — Ukraine hopes to use Russian territory as a bargaining chip.

Russia, relying on North Korean reinforcements, hopes to knock that territory out of Ukraine’s grasp.

“Here, the Russians need to take this territory at any cost, and are pouring all their strength into it, while we are giving everything we have to hold it,” said Sgt. Oleksandr, 46, a leader of a Ukrainian infantry platoon. “We’re holding on, destroying, destroying, destroying — so much that it’s hard to even comprehend.”

He and other soldiers, asking to be identified by only a first name or call sign in accordance with military protocol, said that attacking North Korean infantry had made the battles far more ferocious than before.

“The situation worsened significantly when the North Koreans started arriving,” said Jr. Sgt. Oleksii, 30, a platoon leader. “They are pressuring our fronts en masse, finding weak points and breaking through them.”

Russia, with the help of an estimated 12,000 North Koreans, has retaken about half of the territory it lost over the summer. Its assaults over the past week have further eaten into the territory held by Ukraine.

But Ukrainian forces have also gone on the attack in recent days, seeking to secure an area west of Sudzha, a small town in Russia about 6 miles from the border that has become the anchor for Ukrainian forces, which seized about 200 square miles in August.

The Russians have largely thwarted the assault, but fighting goes on and the situation remains unpredictable, soldiers said.

The renewed fighting comes against a deeply uncertain political backdrop. U.S. President-elect Donald Trump spent months on the campaign trail questioning U.S. military assistance to Ukraine. He has said he wants to bring the war to a swift end, but has not indicated how.

Russian forces have been on the offensive for more than a year in eastern Ukraine, making steady advances despite staggering losses.

With its incursion, Ukraine aims to create a buffer zone to protect hundreds of thousands of civilians in the city of Sumy, less than 20 miles from the border with Russia. Ukraine also wants to ease pressure on the eastern front by drawing Russians back onto their own land.