Antonio Guterres takes the reins of the United Nations on New Year's Day, promising to be a "bridge-builder" but facing an antagonistic incoming US administration led by Donald Trump who thinks the world body's 193 member states do nothing except talk and have a good time.

The former Portuguese prime minister and UN refugee chief told reporters after being sworn-in as secretary-general on December 12 that he will engage all governments "and, of course, also with the next government of the United States" and show his willingness to cooperate on "the enormous challenges that we'll be facing together."

But Trump has shown little interest in multilateralism, which Guterres says is "the cornerstone" of the United Nations, and a great attachment to the Republicans' "America First" agenda.

So as 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 United Nations 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, Trump warned in a tweet: "As to the UN, things will be different after Janusry 20th," the day he takes office.

Trump followed up three days later with another tweet questioning its effectiveness. "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 United Nations, said in an interview with The Associated Press that 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 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," Bolton said, "I think especially in the Trump administration, he would run into big trouble very quickly."

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