WASHINGTON (Reuters) - President Donald Trump complained on Monday that the United States is shouldering an unfair burden of the cost of the United Nations, but said if the world body reforms how it operates, the investment would be worth it.

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Trump, who has frequently criticized the cost to the United States of supporting the NATO alliance, took his concerns directly to the ambassadors of the U.N. Security Council, who joined him at the White House for a lunch.

“If we do a great job, I care much less about the budget because you’re talking about peanuts compared to the important work you’re doing,” Trump told the 15 council envoys.

The United States is the biggest U.N. contributor, paying 22 percent of the $5.4 billion core budget and 28.5 percent of the $7.9 billion peacekeeping budget. These assessed contributions are agreed by the 193-member U.N. General Assembly.

Trump said the U.S. share of those budgets was “unfair.”

He has proposed a 28 percent budget cut for diplomacy and foreign aid, which includes an unspecified reduction in funding for the United Nations and its agencies, as well as enforcement of a 25 percent cap on U.S. funding for peacekeeping operations.

“We need the member states to come together to eliminate inefficiency and bloat and make sure that no one nation shoulders a disproportionate share of the burden,” he said.

Trump’s remarks came as the General Assembly prepares to negotiate in the coming months the U.N. regular budget for both 2018 and 2019 and the peacekeeping budget from July 1, 2017 to June 30, 2018.

United Nations Secretary-General Antonio Guterres met briefly with Trump at the White House on Friday for the first time since both took office earlier this year.

The United States currently owes the United Nations $896 million for its core budget, U.N. officials said. The United States is also reviewing 16 U.N. peacekeeping missions as the annual mandates come up for renewal by the Security Council in a bid to cut costs.

U.N. agencies such as the U.N. Development Program (UNDP), the children’s agency UNICEF, and the U.N. Population Fund (UNFPA), are funded by governments voluntarily.

The State Department said this month it was ending funding for UNFPA, the international body’s agency focused on family planning, as well as maternal and child health in more than 150 countries. Guterres warned that the cut could have “devastating effects” on vulnerable women and girls.

In 2016, the United States was the top contributor to the UNDP’s core budget, with an $83 million donation; the leading donor to UNICEF’s core budget in 2015 with $132 million; and the fourth-largest donor to the UNFPA, giving $75 million.