United States President Donald Trump, in his first appearance at the UN General Assembly, yesterday called for "truly bold reforms" in the United Nations.

"In recent years, the United Nations has not reached its full potential because of bureaucracy and mismanagement," he said, opening a session on reform.

The UN's budget had increased and its staff had more than doubled since the year 2000 but "we have not seen the result of this investment", he said. "We commend the Secretary-General and his call for the United Nations to focus more on people and less on bureaucracy," he added.

Mr Trump has often criticised waste and redundancy at the UN, saying the US pays too much to the agency and does not get enough in return. The US provides 22 per cent of the UN's US$5.4 billion (S$7.2 billion) biennial budget and 28.5 per cent of its US$7.3 billion peacekeeping budget.

A draft declaration on reform circulated by US Ambassador to the UN Nikki Haley stated: "We support the Secretary-General in making concrete changes in the United Nations system to better align its work on humanitarian response, development and sustaining peace initiatives. We commit to reducing mandate duplication, redundancy and overlap, including among the main organs of the United Nations."

All but 73 of the UN's 193 member states had signed it, Ms Haley said at the session, where UN Secretary-General Antonio Guterres also criticised the UN's bureaucracy, "byzantine procedures and endless red tape".

Mr Trump is scheduled to deliver his centrepiece speech to the UN General Assembly today. It will be closely watched for signals of US intentions on the UN, and its policy on North Korea, Iran, Syria and Russia, among others.