The International Air Transport Association said Monday it had suspended Iran Air, the Iranian national airline, from the system for processing international ticket payments as a result of strengthened American economic sanctions against Iran. The suspension severely limits the ability of Iran Air to book passengers abroad. Iran’s civil aviation authority denounced the action as a violation of international rules. The airline association “is not obliged to follow the U.S. embargo against Iran,” said Farhad Parvaresh, the director of the Civil Aviation Organization of Iran.