WASHINGTON, May 20 (Reuters) - The U.S. Treasury said on Thursday it proposed that a global minimum corporate tax rate should be at least 15%, telling an Organization for Economic Cooperation and Development (OECD) tax steering group meeting that it was seeking a higher rate. “Treasury underscored that 15% is a floor and that discussions should continue to be ambitious and push that rate higher,” the department said in a readout of the OECD meeting. (Reporting by David Lawder and Tim Ahmann; Editing by Leslie Adler) Our Standards: The Thomson Reuters Trust Principles.