BRUSSELS, June 9 (Reuters) - European steel industry body Eurofer urged European Union antitrust authorities on Tuesday to look into a planned iron ore joint venture between Rio Tinto and BHP Billiton. "The European steel industry continues to believe that a merger of iron ore assets of this type in a world market already dominated by just three suppliers would not be in the interests of the steel industry," Eurofer said in a statement. (Reporting by Foo Yun Chee; Editing by Dale Hudson)