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 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 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.