A U.S. official reportedly says hundreds of North Korean troops have been killed or wounded fighting alongside Russian forces.

According to Reuters and AFP, the U.S. military official said Tuesday that the North Korean troops have suffered hundreds of casualties in battle against Ukrainian forces in Russia’s Kursk region.

The official, who spoke on condition of anonymity, reportedly said the North Korean soldiers came from the lower ranks as well as those very close to the top.

The U.S. Department of Defense said Monday that it has indications that North Korean troops engaged in combat against Ukraine and suffered casualties. 

According to the Ukrainian government and Russian military bloggers, combat between North Korean troops and Ukrainian forces is intensifying in the Kursk region, where Ukraine began a cross-border incursion in August and still holds parts of the territory.