Antonio Guterres vows to be an active and independent mediator.
Outgoing Secretary-General Ban Ki-moon has farewelled the United Nations after 10-year tenure.
Mr Ban will return to South Korea in January, and is set to consider what role he can play in the future of his country amid a push for him to run for president.
He says he will always have passion for the U-N and his history of helping the defenceless.
