North Korea ratified a defense treaty with Russia, state media reported Tuesday, formally deepening military cooperation that has seen Pyongyang send thousands of troops to help Moscow in 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.

Ukrainian President Volodymyr Zelensky said Russia has deployed nearly 50,000 troops to Kursk, the southern Russian region where Kyiv launched its surprise counteroffensive in the summer.

Ukraine's president says the incursion is limiting the forces Moscow can deploy inside his country.