US President Donald Trump's bid to slash funding for the United Nations would make it “impossible” for it to continue its essential work, a UN spokesman has said, adding that the organisation was ready to discuss reform with Washington.

The Trump proposal cuts about a third from US diplomacy and aid budgets, or nearly $19bn (£14.7bn). This includes cutting some $1bn from UN peacekeeping funding and a steep cut to funding for international organisations.

The United States is the biggest UN contributor, paying 22 per cent of the $5.4bn core budget and 28.5 per cent of the $7.9bn peacekeeping budget. These assessed contributions are agreed by the 193-member UN General Assembly.

“The figures presented would simply make it impossible for the UN 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.

Mr Trump has said the US share of the UN budgets was “unfair.” The General Assembly is currently negotiating the UN regular budget for both 2018 and 2019 and the peacekeeping budget from July 1, 2017 to June 30, 2018.

Mr Trump wants to cap the US peacekeeping contribution at 25 per cent. The United States is reviewing each 16 UN peacekeeping missions as the annual mandates come up for renewal by the Security Council in a bid to cut costs.

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During a lunch with UN Security Council ambassadors at the White House last month, Mr Trump described the US 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,” Mr 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 organisation to pursue our shared goals and values.”

Mr Trump's budget proposal included a 44 per cent cut to funding for international organisations, but does not specify the cuts, other than “funding for organisations that work against US foreign policy interests.”

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

UN agencies such as the UN Development Program (UNDP), the children's agency Unicef, and the UN 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.