North Korean leader Kim Jong Un has supervised a frontline artillery drill simulating an attack on an unspecified South Korean airfield. State media said Friday that the move came as he called for his troops to sharpen their combat readiness in the face of his rivals’ “frantic war preparation moves.” The North Korean report came a day after South Korea’s military detected the North firing at least one short-range ballistic missile toward the sea from a site near the western coastal city of Nampo. That came as South Korea and the U.S. prepare to conduct their biggest joint military training exercise in years.