U.S. President Donald Trump's bid to slash funding for the United Nations would make it "impossible" for it to continue its essential work, a U.N. spokesman said on Wednesday, adding that the organization was ready to discuss reform with Washington.

The Trump proposal cuts about a third from U.S. diplomacy and aid budgets, or nearly $19 billion. This includes cutting some $1 billion from a U.N. peacekeeping funding and a steep cut to funding for international organizations.

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.

"The figures presented would simply make it impossible for the U.N. to continue all of its essential work advancing peace, development, human rights and humanitarian assistance," Stephane Dujarric, spokesman for Secretary-General Antonio Guterres, said in a statement.

Congress sets the federal government budget, and Republicans who control both houses and Democrats have said they do not support such drastic cuts.

Trump has said the U.S. share of the U.N. budgets was "unfair." The General Assembly is currently negotiating the U.N. regular budget for both 2018 and 2019 and the peacekeeping budget from July 1, 2017 to June 30, 2018.

Trump wants to cap the U.S. peacekeeping contribution at 25 percent. The United States is reviewing each 16 U.N. peacekeeping missions as the annual mandates come up for renewal by the Security Council in a bid to cut costs.

During a lunch with U.N. Security Council ambassadors at the White House last month, Trump described the U.S. contributions to the United Nations as "peanuts compared to the important work" as he pushed the world body to reform.

"The Secretary-General is totally committed to reforming the United Nations," Dujarric said. "We stand ready to discuss with the United States and any other member state how best we can create a more cost-effective organization to pursue our shared goals and values."

Trump's budget proposal included a 44 percent cut to funding for international organizations, but does not specify the cuts, other than "funding for organizations that work against U.S. foreign policy interests."

The State Department said last month it was ending funding for the U.N. Population Fund, an agency focused on family planning, maternal and child health in more than 150 countries. Guterres warned the cut could have "devastating effects."

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.

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.