The United States is concerned by Russia's growing relationship with North Korea and by what technology the two countries may be exchanging, the top U.S. arms control official said on Wednesday.

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 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.

"Today I can confirm that over 10,000 (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,

South Korea's government said Thursday "effective and phased" measures will be taken over North Korea's dispatch of troops to fight for Russia against Ukraine, after the nation's spy agency confirmed that the North's soldiers have entered combat in Russia's Kursk.

North Korean troops have officially engaged in combat alongside Russian forces in Ukraine's Kursk region, marking a significant development.