United Nations Secretary-General Ban Ki-moon (L) and Secretary-General-designate Mr. Antonio Guterres stand together after the swearing-in ceremony at UN headquarters in New York, U.S., December 12, 2016. REUTERS/Lucas Jackson - RTX2UP30

UN's new secretary-general sworn in

Former Portuguese prime minister Antonio Guterres was sworn in Monday as Secretary-General of the United Nations, becoming the ninth UN chief in the body's 71-year history.