President Donald Trump started his week at the United Nations on Monday by criticizing it for underperformance and pledging to make it more effective in reaching its mission.

Trump, who is scheduled to address the General Assembly on its opening day Tuesday, praised some U.N. initiatives but contended that the international body has failed to produce results in line with the investment in it.

"In recent years, the United Nations has not reached its full potential because of bureaucracy and mismanagement," Trump said Monday morning in his first U.N. appearance as president. "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. But I know under the secretary-general that's changing, and it's changing fast."

Trump's comments follow his pledges as a candidate to rely less on international organizations and multilateral solutions in foreign policy. The president has previously called for more U.S. policymaking on a bilateral basis and criticized organizations like the U.N. and North Atlantic Treaty Organization.

On Monday, Trump argued that the U.N. should "focus more on people and less on bureaucracy."

More than 120 countries attended the Monday morning meeting about reforming the U.N., according to Reuters. The nations signed on to a declaration, drafted by the United States, putting their weight behind Secretary-General Antonio Guterres' reform efforts.

Trump apparently believes at least one thing in the U.N.'s vicinity is living up to its potential. The president made a reference to his Trump World Tower, which sits across from the U.N. building, at the start of his remarks.

"I actually saw great potential across the street and it was only for the reason that the United Nations was here that that turned out to be such a successful project," Trump said.

The president has bilateral meetings with Israeli Prime Minister Benjamin Netanyahu and French President Emmanuel Macron on Monday.

— Reuters contributed to this report