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 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.

North Korea has signed off an a treaty with Russia calling for a "comprehensive strategic partnership," Pyongyang's state-controlled news agency KCNA reported on Tuesday. It comes as North Korean troops are in Russia fighting against Ukraine,

North Korea has ratified a landmark defence pact with Russia, state media reported Tuesday, deepening security cooperation that has seen Pyongyang send thousands of troops to help Moscow fight Ukraine.