US President Donald Trump warned "bureaucracy" is stopping the United Nations from realizing its potential Monday, a barbed message during his first appearance at an institution he once derided as a talking shop.

Attending a discussion on UN reform, Trump insisted he had always seen the "great potential" of the organization from a perch "right across the street" at his New York home.

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