US President Donald Trump on Monday warned "bureaucracy" is holding the United Nations back, a barbed message during his first appearance at an institution he once derided as a talking shop.

Kicking off a frantic week of diplomacy with a panel discussion on UN reform, Trump noted a personal history with the New York-based institution.

He had seen "great potential right across the street" from UN headquarters, Trump said, referencing his nearby 72-floor residential skyscraper Trump World Tower.

"To be honest with you, and it was only for the reason that the United Nations was here that that turned out to be such a successful project," the businessman-turned-president told delegates.

But Trump also warned that the United States -- a founding member of the UN and its biggest financial contributor -- wanted a better return on its investment.

"The United Nations was founded on truly noble goals" he said, adding that while progress has been made "in recent years the United Nations has not reached its full potential, because of bureaucracy and mismanagement."

"While the United Nations on a regular budget has increased by 140 percent, and its staff has more than doubled since 2000, we are not seeing the results in line with this investment."

He called on the institution to "focus on results," a call that was echoed by UN Secretary General Antonio Guterres, who said a reformed UN needs to focus "more on people and less on process."