US President Donald Trump's bid to slash funding for the United Nations would make it "impossible" for it to continue its essential work, according to a spokesman for the UN chief.

The Trump proposal cuts about a third from US diplomacy and aid budgets, or nearly $19bn. This includes cutting some $1bn from a UN peacekeeping funding and a steep reduction to his country's contribution for international organisations.

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The US is the biggest UN contributor, paying 22 percent of the $5.4bn core budget and 28.5 percent 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 on Wednesday.

"We stand ready to discuss with the US and any other member states how best we can create a more cost-effective organisation to pursue our shared goals and values."

Trump, who wants to shift money towards the military with a $54bn increase in defence spending, has previously said the US share of the UN budgets was "unfair".

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The UN's General Assembly is currently negotiating the regular budget for both 2018 and 2019 and the peacekeeping budget from July 1, 2017 to June 30, 2018.

The budget proposal says Trump wants to cap the US peacekeeping contribution at 25 percent but notes that the UN General Assembly only revises assessment rates every three year. The next negotiation is not due until 2018.

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

'Devastating effects'

Trump's budget proposal said the US would cut funding for UN peacekeeping by about $1bn, or 50 percent. It also included a 44 percent contributing reduction to funding for international organisations, but does not specify the cuts, other than "funding for organisations that work against US foreign policy interests".

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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. 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 US was the top contributor to the UNDP's core budget, with an $83m donation; the leading donor to UNICEF's core budget in 2015 with $132m; and the fourth-largest donor to the UNFPA, giving $75m.

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Atul Khare, the UN undersecretary-general for peacekeeping support, said at a news conference that peacekeeping "is effective", pointing to nearly 70 missions that have wrapped up and "left a legacy of stability in countries spanning from El Salvador to Namibia to East Timor".

The UN has already announced that three missions will soon be closing in Ivory Coast, Liberia and Haiti.

Khare said "peacekeeping is cost effective", saying the current $7.8bn budget supports 16 missions, a regional center and logistics base and the deployment of over 113,000 personnel. In addition, he said, the UN supports 22,000 African Union peacekeepers and 595 civilians in Somalia.

Adjusting for inflation, Khare said, "the cost of UN peacekeeping to member states today is 17 percent lower in 2016-17 than it was in 2008-09 when measured as cost per capita of deployed uniformed personnel".