United States (US) President Donald Trump made his debut at the United Nations on Monday, using his first moments at the world body to urge the 193-nation organisation to reduce bureaucracy and costs while more clearly defining its mission around the world.

But while Trump chastised the United Nations, an organisation he sharply criticised as a presidential candidate for its spiralling costs, he said the United States would “pledge to be partners in your work” in order to make the UN “a more effective force” for peace across the globe.

“In recent years, the United Nations has not reached its full potential due to bureaucracy and mismanagement,” said Trump, who rebuked the United Nations for a ballooning budget. “We are not seeing the results in line with this investment.”

The president pushed the UN to focus “more on people and less on bureaucracy” and to change “business as usual and not be beholden to ways of the past which were not working.”

He also suggested that the US was paying more than its fair share to keep the New York-based world body operational.

But he also complimented steps the United Nations had taken in the early stages of the reform process and made no threats to withdraw his nation's support.

His measured tone stood in stark contrast to his last maiden appearance at a global body, when he stood at Nato's new Brussels headquarters in May and scolded member nations for not paying enough and refusing to explicitly back its mutual defence pact.

While running for office, Trump labelled the UN as weak and incompetent, and not a friend of either the United States or Israel.

But he has softened his tone since taking office, telling ambassadors from UN Security Council member countries at a White House meeting that the UN has “tremendous potential.”

Trump more recently has praised a pair of unanimous council votes to tighten sanctions on North Korea over its continued nuclear weapons and ballistic missile tests.

Trump's big moment comes Tuesday, when he delivers his first address to a session of the UN General Assembly.

The annual gathering of world leaders will open amid serious concerns about Trump's priorities, including his policy of “America First,” his support for the UN and a series of global crises. It will be the first time world leaders will be in the same room and able to take the measure of Trump.

The president on Monday praised UN Secretary-General Antonio Guterres, who also spoke at the reform meeting and said he shared Trump's vision for a less wasteful UN to “live up to its full potential.”

The US has asked member nations to sign a declaration on UN reforms, and more than 120 have done so. The president also kicked off his maiden speech at the world body by referring to the Trump-branded apartment tower across First Avenue from the UN.

Nikki Haley, the US ambassador to the UN, said Trump's criticisms were accurate at the time, but that it is now a “new day” at the UN.

An organisation that “talked a lot but didn't have a lot of action” has given way to a “United Nations that's action-oriented,” she said, noting the Security Council votes on North Korea this month.

Guterres has proposed a massive package of changes, and Haley said the UN is “totally moving toward reform.”

Trump riffed on his campaign slogan when asked about his main message for the General Assembly.

“I think the main message is 'make the United Nations great.' Not again, 'make the United Nations great,'” Trump said as he left the UN building. “Such tremendous potential, and I think we'll be able to do this.”

Trump also planned separate talks Monday with Israeli Prime Minister Benjamin Netanyahu and French President Emmanuel Macron. US national security adviser HR McMaster said “Iran's destabilizing behaviour” would be a major focus of those discussions. He also was having dinner with Latin American leaders.

The United States is the largest contributor to the UN budget, reflecting its position as the world's largest economy. It pays 25 per cent of the UN's regular operating budget and over 28 percent of the separate peacekeeping budget a level of spending that Trump has complained is unfair.

The Trump administration is conducting a review of the UN's 16 far-flung peacekeeping operations, which cost nearly $8 billion a year. Cutting their costs and making them more effective is a top priority for Haley.