US President Donald Trump has said that the United Nations is not living up to its potential because of bureaucracy and mismanagement.
In his first speech at the UN in New York, he said: "Focus more on people, less on bureaucracy."
Mr Trump, who has criticised what he sees as a disproportionate contribution by the US, said the UN could become stronger if all worked together.
The US funds 22% of the UN's regular budget and 28% of UN peacekeeping.
Mr Trump is due to deliver a longer speech when he addresses the UN General Assembly for the first time on Tuesday. He is expected to call for a harder line on North Korea and Iran.
At a special meeting on UN reform on Monday, he encouraged member states to take a "bold stand" to change the UN's "business-as-usual" approach rather than "be beholden to ways of the past which are not working".
"I am confident that if we work together and champion truly bold reforms the United Nations will emerge as a stronger, more effective, more just and greater force for peace and harmony in the world."
What else is Trump saying?
- Israel and the Palestinians: After meeting the Israeli Prime Minister Benjamin Netanyahu, Mr Trump said there was a "good chance" of a peace agreement. "So we're working very hard on it. We'll see what happens. Historically, people say it can't happen. I say it can happen."
- Iran: The president is critical of the 2015 deal between Iran, the US and other world powers, including China and Russia, designed to prevent Iran from developing a nuclear weapon. Mr Trump said he would unveil what he wanted to do "very soon".
- North Korea: The crisis over the country's weapons programme is expected to be a major part of his address to the UN on Tuesday. In a phone call, Mr Trump and Chinese President Xi Jinping committed to "maximising pressure" on the North through "vigorous enforcement" of UN Security Council resolutions, the White House said.
- Global warming: The White House reaffirmed on Monday President Trump's decision that the US will pull out of the UN-brokered Paris accord on climate change. He met French President Emmanuel Macron, who has strongly defended the climate agreement.
- 4 July celebrations: Before meeting Mr Macron, Mr Trump said he planned to transform the Independence Day celebrations in Washington into a military parade inspired by France's Bastille Day. "We're actually thinking about Fourth of July, Pennsylvania Avenue, having a really great parade to show our military strength." Mr Trump, who was in Paris for this year's Bastille Day, said the parade was a "beautiful thing to see".
While still a candidate for the US presidency, Mr Trump sharply criticised the UN, speaking of its "utter weakness and incompetence".
On Monday, he called on Secretary General António Guterres to make changes, while also praising him for making progress on some of the problems he spelled out. Mr Guterres responded by agreeing that excessive red tape kept him up at night.
"Someone out to undermine the UN could not have come up with a better way to do it than by imposing some of the rules we have created ourselves," said the Portuguese diplomat.
"I even sometimes ask myself whether there was a conspiracy to make our rules exactly what they need to be for us not to be effective," he added.
Only a mild rebuke
By Laura Trevelyan, BBC News, New York
There was some trepidation here at UN HQ ahead of President Trump's speech on reform of the world body. Hurricane Trump is about to make landfall, as one UN source told me, reflecting the uncertainty about what the unpredictable Donald Trump might say, as America First came to the heart of multilateralism.
In the end, this was a speech which contained only a relatively mild rebuke (by Trump standards) of the UN's bureaucracy and mismanagement - and warm words about the UN's potential and how the US will be a partner in the reform road ahead.
This was an encounter which the 38th floor of UN HQ, where Secretary General Gutierrez resides, can feel pleased with - no tongue-lashing for being an obsolete talking shop.
And after all, let's not forget that the UN Security Council has proved rather useful to President Trump, passing two sanctions resolutions against North Korea in punishment for its nuclear programme.
President Trump also complained that the US was "not seeing results in line with US investment".
"We must ensure that no one and no member state shoulders a disproportionate share of the burden - and that's militarily or financially."
Under pressure from the Trump administration, the UN has already cut its budget by more than $500m (£370m).
Trump's past UN tweets
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!— Donald J. Trump (@realDonaldTrump) December 26, 2016
The cheap 12 inch sq. marble tiles behind speaker at UN always bothered me. I will replace with beautiful large marble slabs if they ask me.— Donald J. Trump (@realDonaldTrump) October 3, 2012