The Defense Department on Monday said about 10,000 North Korean military troops have deployed inside Russia’s occupied Kursk region and are making their way to the front lines to help Moscow in its war against neighboring Ukraine. 

NATO Secretary General Mark Rutte called on Moscow and Pyongyang to cease the military cooperation, which he said was a threat to stability and security in both Europe and East Asia.

Pentagon officials said they didn’t know if the North Korean troops were expected to fight as part of their own units or fill holes in Russian Army companies and battalions that have been decimated by the relentless pace of the war. About 2,000 North Korean soldiers remain on Russian but could soon find themselves heading to Kursk, officials said, in the largest overseas deployment of troops by Pyongyang since the Korean War.

Maj. Gen. Pat Ryder, a Defense Department spokesman, couldn’t confirm reports that some North Korean troops have already taken part in combat missions on Russia’s behalf inside Kursk.

“Should those troops engage in combat support operations against Ukraine, they would become legitimate military targets,” Gen. Ryder said.

Russian forces trained their North Korean allies in basic infantry tactics such as clearing trenches, artillery operations and the use of armed drones which have proven particularly potent on the battlefield in Ukraine.

“The Kremlin has also provided these [North Korean] troops with Russian uniforms and equipment, and all of that strongly indicates that Russia intends to use these foreign forces in front-line operations in its war of choice against Ukraine,” Defense Secretary Lloyd Austin said last week.

Kyiv launched a surprise incursion into Russia’s Kursk region about three months ago. Pentagon officials said Moscow’s immediate response to the incursion was muddled and they have been unable so far to eject Ukrainian forces.

An additional 10,000 to 12,000 North Korean troops on Russia’s side could be significant, depending on how well the North Korean personnel are integrated into the overall operation in Kursk, Pentagon officials said.

“But, the Ukrainians are battle-hardened veterans who know how to fight. Every indication is that they will continue to defend Ukrainian sovereignty and continue to defend … the [Russian] territory that they’ve taken,” Gen. Ryder said. “We’ll see how that plays out.”