Ban Ki-moon ends 10 years at the helm of the United Nations lamenting the "fires still burning" from Syria to South Sudan but buoyed by a global agreement to combat climate change and new UN goals to fight poverty and inequality.

As a final act before his term ends at midnight on New Year's Eve, the secretary-general will push the button starting the descent of the glittering 11,875-pound ball in New York's Times Square in the countdown to 2017's arrival.

At that moment, former Portuguese prime minister Antonio Guterres will start his tenure as United Nations chief for the next five years.

Looking back at his stewardship of the United Nations at a farewell news conference earlier this month, Ban told reporters "this has been a decade of unceasing test." 

While he has seen collective action improve millions of lives, Ban expressed frustration at the failure to end Syria's war, now in its sixth year, and conflicts in South Sudan, Yemen, Central African Republic and Congo, to name a few.

And in rare criticism of world leaders, he blamed unnamed presidents, prime ministers and monarchs for the turmoil in the world on Saturday and expressed disappointment many care more about retaining power than improving their people's lives. He singled out Syria, saying he can't understand why it is being held hostage to "the destiny" of one man, Bashar Assad.

Even after leaving the UN, Ban said he will keep urging new and longstanding leaders to embrace the "pre-eminent 21st century fact" that "international cooperation remains the path to a more peaceful and prosperous world" and to demonstrate "compassionate leadership." 

To reinforce this, the secretary-general's final trip this month was to visit the Abraham Lincoln Presidential Library and Museum in Springfield, Illinois. "Lincoln was a heroic force for equality, integration and reconciliation; and desperately, we need that spirit today," Ban said of the US leader during America's Civil War.

Ban has also expressed frustration at the way the UN operates and expectations in some quarters that the secretary-general has the power "to be some almost almighty person." 

That's impossible, he told the AP in September, because the UN's 193 member states make decisions and the secretary-general implements them. The UN chief cannot implement his or her own policies and initiatives.

John Bolton, who was US ambassador to the UN when Ban was selected to be secretary-general, said President George W Bush's administration supported him because "we wanted someone who would do what the member governments wanted" and not take the lead on issues and act as the world's top diplomat like then-secretary-general Kofi Annan.

"I think Ban Ki-moon lived up to our expectations, which is not to say I agreed with every position he took on climate change and things like that," Bolton told AP.