United Nations (AFP) - The United States is seeking a US$250 million (S$338 million) cut to the core budget of the United Nations for 2018-2019, on top of US$200 million in savings already proposed by UN Secretary-General Antonio Guterres, diplomats said on Tuesday.

The budget is under negotiations in a General Assembly committee which is expected to adopt it before the end of December.

Guterres has proposed capping the UN biennial budget at US$5.4 billion, shaving off US$200 million from the 2016-2017 budget.

Two diplomats told AFP that the United States had put forward a proposal for an additional US$250 million reduction - a five-per cent-cut that another diplomat said would be nearly impossible to achieve.

The United States is the UN's number one financial contributor, providing 22 per cent of the core budget.

Diplomats said the proposed US budget cuts could target political missions in Libya and Afghanistan as well as the office for Palestinian rights and communication services.

The UN mission to the United Nations declined to comment. There was no immediate UN reaction.

The European Union has also put forward a proposal to seek an additional $170 million in savings from the budget.

"This is a classic game," said a diplomat.

"There's always two camps - the United States and the European Union - who want to reduce the budget and the rest of the countries who don't."

The UN's operating budget is separate from its peacekeeping budget, which was cut by US$600 million this year, under pressure from the US administration of President Donald Trump.

The push for deeper cuts comes as Guterres is trying to build support for his plans for reform of the UN bureaucracy.

During a meeting held on the sidelines of the annual General Assembly debate in September, Trump said the United Nations had failed to reach its "full potential due to bureaucracy and mismanagement."

"We are not seeing the results in line with this investment," he said.

The United Nations employs 40,000 people recruited worldwide.