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.

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 ratified a mutual defence treaty with Russia signed by the two countries' leaders in June, which calls for each side to come to the other's aid in case of an armed attack, state media KCNA said on Tuesday.