The new United Nations chief, Antonio Guterres, warned on Tuesday that the world body faces "very challenging times" and asked for support of reforms to make it better able to deal with them.

Before beginning his first day of work at UN headquarters, Guterres, who succeeded Ban Ki-moon as secretary-general on Sunday, addressed staff and diplomats about the need for an overhaul of the international organization.

"I think it is useful to say there are nomiracles," he said. "I am sure I am not a miracle maker."