U.S. 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 organization to reduce bureaucracy and costs while more clearly defining its mission around the world.

But while Trump chastised the United Nations — an organization he sharply criticized as a candidate for president 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 United States was paying more than its fair share to keep the organization operational.

But he also complimented the 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 the member nations for not paying enough and refusing to explicitly back its mutual defence pact.

UN Secretary General Antonio Guterres acknowledged that while the UN has been weighed down by its own rules, the goal of reform is to make the organization "more nimble, effective, flexible and efficient," he said on Monday.

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.

Support for reforms

The U.S. has asked member nations to sign the declaration on UN reforms, and more than 120 have done so. Trump wants the UN to cut spending and make other operational changes.

Nikki Haley, the U.S. 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 organization 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.

United States Ambassador to the United Nations Nikki Haley said prior to this week's meetings that 'it's a new day at the UN.' (Seth Wenig/Associated Press)

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

"We said that we needed to get value for our dollar and what we're finding is the international community is right there with us in support of reform. So it is a new day at the UN," she said Sunday on CNN's State of the Union. She said Trump's pleas had been heard and "what we'll do is see him respond to that."

Talks with world leaders planned

The speech began a busy week of diplomacy for Trump, who is slated to meet separately with more than a dozen world leaders along the sidelines of the UN. His first bilateral meeting was Monday with Israeli Prime Minister Benjamin Netanyahu during which he declared that they "are giving it an absolute go"' in Middle East peace talks.

Israeli PM talks Middle East peace, Iranian nuclear deal 1:06

Trump also planned to hold separate talks Monday with French President Emmanuel Macron. U.S. national security adviser H.R. McMaster said the conversations would be wide-ranging, but that "Iran's destabilizing behaviour" would be a major focus of Trump's discussions with both leaders.

Trump told Jewish leaders on a conference call last week that his team is working hard to achieve a peace agreement between the Israelis and Palestinians and that he hoped to see "significant progress" on a deal before the end of the year. Trump is scheduled to meet later this week with Palestinian Authority President Mahmoud Abbas.

Breakthroughs on a Middle East peace agreement are not expected. Jared Kushner, Trump's son-in-law and senior adviser on the issue, recently returned from a trip to the Middle East.

Trump was also hosting a dinner for Latin American leaders. Venezuela, which has been gripped by economic and political turmoil, will be discussed, McMaster said.

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 per cent of the separate peacekeeping budget a level of spending that Trump has complained is unfair.

"We need the member states to come together to eliminate inefficiency and bloat, and to ensure that no one nation shoulders a disproportionate share of the burden militarily or financially," Trump told the Security Council ambassadors as they dined at the White House in April. "This is only fair to our taxpayers."

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