U.S. President Donald Trump on Monday debuted at the United Nations and called for its reform, criticizing the world body over "bureaucracy and mismanagement."

"In recent years, the United Nations has not reached its full potential because of bureaucracy and mismanagement," Trump told a high-level meeting on U.N. reform in New York.

Citing a 140 percent increase in the U.N. regular budget and the more than doubling of its staff since 2000, Trump said, "We are not seeing the results in line with this investment."

"We seek a United Nations that regains the trust of the people around the world," he said. "The United Nations must hold every level of management accountable, protect whistleblowers, and focus on results rather than on process."

The businessman-turned-president, who took office in January with his "America First" mantra, said that to honor the people of the United States, the United Nations must ensure that no member state shoulders "a disproportionate share of the burden," both financially and militarily.

As leader of the biggest contributor to the U.N. budget and peacekeeping operations, Trump is scheduled on Tuesday to deliver his first general debate address at the U.N. General Assembly.

World leaders will be closely watching Trump's speech amid concerns about his support for global institutions such as the United Nations and U.S. leadership in global and regional issues following Washington's withdrawal from the Paris accord on climate change and the Trans-Pacific Partnership trade deal.

In Monday's remarks, Trump said his administration backs U.N. Secretary General Antonio Guterres' efforts to "look across the entire system" and to find ways the United Nations can be better at development, management, peace and security.

"We encourage the secretary general to fully use his authority to cut through the bureaucracy, reform outdated systems and make firm decisions to advance the U.N.'s core mission," he said.