The United Nations on Thursday warned that its operations could suffer if the United States were to slash funding to the world body as called for in President Donald Trumps first budget proposal.

Secretary-General Antonio Guterres "is totally committed to reforming the United Nations and ensuring that it is fit for purpose and delivers results in the most efficient and cost-effective manner," spokesman Stephane Dujarric told reporters.

"However, abrupt funding cuts can force the adoption of ad hoc measures that will undermine the impact of longer-term reform efforts."

Guterres "stands 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," said the spokesman.

The secretary general, who took office at the start of the year, has met several times with Washington's ambassador to the UN, Nikki Haley, he added.

Trump's budget proposal, unveiled Thursday, calls for a cut in US funding to the UN, and says Washington "would not contribute more than 25 percent for UN peacekeeping costs."

When asked about the budget cuts, France's ambassador to the UN, Francois Delattre, told reporters that the world more than ever needed "a strong UN and an America that stays committed to world affairs."

"America's retreat and unilateralization or even the perception of it by other players would create the risk of coming back to the old 'spheres of influence' policy, and history teaches us that it has only led to more instability," Delattre added.