Kim Jong Un, the ruler of North Korea, visited a Russian fighter jet plant on Friday as part of a visit that Washington and its allies worry will help Pyongyang’s missile programme and strengthen Russia’s military presence in Ukraine. The factory is subject to Western sanctions. When Kim and Russian President Vladimir Putin met on Wednesday, they talked about military issues, the conflict in Ukraine, and advancing bilateral collaboration. In a statement released on Friday, South Korea and the United States warned that any military collaboration between North Korea and Russia would be punished in accordance with UN sanctions imposed...