Air supremacy allows increased bombing efforts, tactical air support for ground forces, paratroop assaults, airdrops and simple cargo plane transfers, which can move ground forces and supplies. Air power is a function of the degree of air superiority and numbers or types of aircraft, but it represents a situation that defies black-and-white characterization. NATO forces in air superiority over Kosovo have lost a stealth strike aircraft to an "obsolete"[1] Serbian air defense system, and primitive An-2 biplanes (less visible to radar than metal planes) were considered[according to whom?] for some time a serious capability of the Korean People's Air Force in North Korea.[citation needed]