South Korea’s National Intelligence Service (NIS) said Wednesday that North Korean troops sent to Russia as mercenaries have moved to the frontlines in Kursk over the past two weeks and are “already engaging in combat” against Ukrainian forces.

Kursk is the province in Russia invaded by Ukrainian troops in August, catching the Russian military by surprise. The Ukrainians seemingly hoped Russia would divert troops away from its invasion force to recapture Kursk, but instead the Russians turned to their “strategic partners” in North Korea for about 10,000 troops.

On Tuesday, North Korean dictator Kim Jong-un ratified a “comprehensive strategic partnership” treaty with Russia, the most important defense pact signed by the two countries since the end of the Cold War. Among other provisions, it calls on each country to render immediate military assistance if the other is attacked.

“Today I can confirm that over 10,000 DPRK [North Korean] soldiers have been sent to eastern Russia, and most of them have moved to the far western Kursk Oblast, where they have begun engaging in combat operations with Russian forces,” State Department spokesman Vedant Patel said on Tuesday, providing the first official U.S. confirmation that North Korea’s soldiers are fighting the Ukrainians.

“Russian forces have trained the DPRK soldiers in artillery, in UAV and basic infantry operations, including trench clearing, which are critical skills for frontline operations,” Patel said.

During a meeting with North Atlantic Treaty Organization (NATO) Secretary-General Mark Rutte in Brussels, outgoing U.S. Secretary of State Antony Blinken confirmed that North Korean troops are “now quite literally in combat.”

Blinken said Russia’s use of North Korean troops “demands, and will get, a firm response.”

Rutte also said NATO has seen “North Koreans being active in Ukraine.”

“These North Korean soldiers present an extra threat to Ukraine and will increase the potential for Putin to do harm,” he said.

Ukrainian troops have expressed their contempt for the North Korean forces, boasting of learning just enough Korean to tell them how to surrender and predicting the mercenaries would “die uselessly.”