The U.S. said Wednesday that 3,000 North Korean troops have deployed to Russia and are training at several locations, calling the move very serious and warning that those forces will be “fair game” if they go into combat in Ukraine.

North Korea’s dispatch of 1,500 special forces to Russia, as the vanguard of an estimated 12,000 troops to join Moscow’s invasion of Ukraine, marks a significant evolution of Pyongyang’s evolving strategic partnership with Moscow.