North Korea sent its best troops to aid Russia in its war against Ukraine. But after months of suffering severe losses, they have been taken off the front line.

North Korean troops have not been seen on the frontlines in Russia’s Kursk region for several weeks, a Ukrainian military official said Friday, amid reports of mass casualties among Pyongyang’s forces.

North Korean forces have been pulled off the frontlines in the Kursk region after suffering heavy losses in combat, The New York Times reports. According to Ukrainian and US officials, North Korean soldiers who had been fighting alongside Russian forces against Ukraine were withdrawn after sustaining significant casualties.