The U.S. Federal Trade Commission filed a lawsuit against Qualcomm Inc on Tuesday, accusing the company of using "anticompetitive" tactics to maintain its monopoly on a key semiconductor used in mobile phones. The FTC, which works with the Justice Department to enforce antitrust law, said that San Diego-based Qualcomm used its dominant position as a supplier of certain phone chips to impose "onerous" supply and licensing terms on cellphone manufacturers and to weaken competitors. Qualcomm said in a statement that it would "vigorously contest" the complaint and denied FTC allegations that it threatened to withhold chips in order to collect unreasonable licensing fees.