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.

North Korean soldiers deployed to the Kursk region of Russia have begun combat operations against Ukrainian troops, the U.S. State Department confirmed Tuesday.

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.

North Korea ratified a major defence pact with Russia on Tuesday, outlining mutual military aid and requiring both countries to provide each other with immediate military assistance if attacked. Russia had already completed its side of the ratification last week after the pact was signed by Russian President Vladimir Putin and North Korean leader Kim Jong Un.

The United States confirmed Tuesday that North Korean troops, who have been deployed to Russia's western front-line Kursk region, have begun engaging in combat operations against Ukrainian forces.

North Korea has formalized a mutual defense treaty with Russia, according to state media reports on Nov. 12. The agreement commits both nations to come to each other's aid in the event of an armed attack.