Antonio Guterres has begun the first day as the United Nations' new secretary general with a New Year appeal to the world: "Let us resolve to put peace first."

The former Portuguese prime minister and UN refugee chief made the plea just after taking over the reins of the United Nations from Ban Ki-moon, whose second five-year term ended at midnight on December 31.

Mr Guterres said the only way to help "the millions of people caught up in conflict, suffering massively in wars with no end in sight" was for people, governments and leaders to "strive to overcome our differences".

"All that we strive for as a human family - dignity and hope, progress and prosperity - depends on peace," he said.

Mr Guterres has said he will engage all governments "and, of course, also with the next government of the United States" and show his willingness to co-operate on "the enormous challenges that we'll be facing together".

But US president-elect Donald Trump has shown little interest in multilateralism, which Mr Guterres contends is "the cornerstone" of the UN and great attachment to the Republicans' "America First" agenda, saying the world body's 193 member states do nothing except talk and have a good time.

As Mr Guterres begins his five-year term facing conflicts from Syria and Yemen to South Sudan and Libya and global crises from terrorism to climate change, US support for the UN remains a question mark.

And it matters because the US is a veto-wielding member of the UN Security Council and pays 22 per cent of the UN's regular budget and 25 per cent of its peacekeeping budget.

Immediately after the United States allowed the security council to condemn Israeli settlements in the West Bank on December 23 in a stunning rupture with past practice, Mr Trump warned in a tweet: "As to the UN, things will be different after Jan 20th" - the day he takes office.

Mr Trump followed up three days later with another tweet questioning its effectiveness, saying: "The United Nations has such great potential but right now it is just a club for people to get together, talk and have a good time. So sad!"

John Bolton, a conservative Republican and former US ambassador to the UN, said Mr Guterres would be well advised "especially given the incoming Trump administration" to follow the model of his predecessor Ban Ki-moon and do what member governments want.

If he tries to follow what Mr Ban's predecessor, Kofi Annan, did as secretary general and try to be the world's top diplomat and what some called "a secular pope", Mr Bolton said: "I think especially in the Trump administration, he would run into big trouble very quickly."

Mr Guterres has made clear that his top priority will be preventing crises and promoting peace.

In the first minute after taking over as UN chief on Sunday, he issued an "appeal for peace", urging all people in the world to make a shared New Year's resolution to put peace first.

"Let us make 2017 a year in which we all - citizens, governments, leaders - strive to overcome our differences," he said.

He has said there is enormous difficulty in solving conflicts, a lack of "capacity" in the international community to prevent them and the need to develop "the diplomacy for peace", which he plans to focus on.

Mr Guterres has said he will also strive to deal with the inequalities that globalisation and technological progress have helped deepen, creating joblessness and despair, especially among the world's youth.

"Today's paradox is that despite greater connectivity, societies are becoming more fragmented. More and more people live within their own bubbles, unable to appreciate their links with the whole human family," he said after his swearing-in.

Mr Guterres said the values enshrined in the UN Charter that should define the world that today's children inherit - peace, justice, respect, human rights, tolerance and solidarity - were threatened, "most often by fear".

"Our duty to the peoples we serve is to work together to move from fear of each other, to trust in each other, trust in the values that bind us, and trust in the institutions that serve and protect us," he said.

"My contribution to the United Nations will be aimed at inspiring that trust."

Mr Guterres won the UN's top job after receiving high marks from almost every diplomat for his performance in the first ever question-and-answer sessions in the General Assembly for the 13 candidates vying to replace Mr Ban, whose second five-year term ended at midnight on December 31.

In an interview during his campaign Mr Guterres said the role of secretary general should be "an honest broker, a consensus builder" who engages as much as possible, in many circumstances discreetly.

"It's not just to have a personal agenda, because it would be regrettable or ineffective, or to appear in the limelight. No. On the contrary, it's to act with humility to try to create the conditions for member states that are the crucial actors in any process to be able to come together and to overcome their differences," he said.

Whether the Trump administration will join him and UN efforts to tackle what he sees as "a multiplication of new conflicts" and the myriad problems on the global agenda remains to be seen.

Mr Guterres also wants to reform the UN to make it "nimble, efficient and effective", saying it must "focus more on delivery and less on process, more on people and less on bureaucracy" and ensure the more than 85,000 UN staff working in 180 countries are being used effectively.