US president Donald Trump pledged his country's support to the United Nations in his first meeting - despite strong criticism when he was a candidate for the White House.
Mr Trump chaired a session aimed at reforming the world body alongside UN Secretary-General Antonio Gutteres
He urged the UN to cut what he deemed its bloated bureaucracy and fulfil its mission.
"In recent years, the United Nations has not reached its full potential due to bureaucracy and mismanagement.
"We are not seeing the results in line with this investment."
The president urged 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 the US was paying more than its fair share to keep the New York-based world body operational.
The short remarks at a forum on UN reforms were a precursor to today's main event, when Mr Trump will address the UN General Assembly for the first time, a speech nervously awaited by world leaders concerned about what the president's "America first" vision means for the future of the world body.
Mr Trump riffed on his campaign slogan when asked to preview his central message to the General Assembly, saying: "I think the main message is 'make the United Nations great' - not 'again.' 'Make the United Nations great'.
"Such tremendous potential, and I think we'll be able to do this," he added.
But even as the president chastised the UN, he pledged that the United States would be "be partners in your work" to make the organisation a more effective force for peace across the globe.
He praised the UN's early steps toward reform and made no threats to withdraw US support.
The president's more measured tone stood in sharp contrast to the approach he took at NATO's new Brussels headquarters in May, when he scolded member nations for not paying enough and refused to explicitly back its mutual defence pact.
While running for office, Mr Trump had labelled the UN as weak and incompetent, and not a friend of either the United States or Israel.
But he has softened his message since taking office, telling ambassadors at a White House meeting in April that the UN has "tremendous potential."
Mr Trump more recently has praised a pair of unanimous UN Security Council votes to tighten sanctions on North Korea over its continued nuclear weapons and ballistic missile tests.
The annual gathering of world leaders opens amid serious concerns about Mr Trump's priorities. For many world leaders, it will be their first chance to take the measure of the president in person.