North Korean troops have begun engaging in combat operations alongside Russian forces, the U.S. State Department said on Tuesday, expressing concern that Russia had turned to North Korea for soldiers to fight in its war with Ukraine.

Nearly 10,000 North Korean troops have arrived in Russia near Ukraine ’s border, sparking concern that they may soon join Moscow’s war effort. US secretary of state Antony Blinken confirmed the deployment on Thursday, adding that these troops have been trained in artillery and drone operations and basic infantry tactics.

North Korea ratified a major defense treaty with Russia stipulating mutual military aid, the North’s state media reported Tuesday, as the U.S., South Korea and Ukraine say North Korea has sent thousands of troops to Russia to support its war against Ukraine.