The United Nations (UN) has warned that President Donald Trump's plans to cut contributions to peacekeeping will make such work "impossible".
The US administration signalled heavy cuts to UN operations, in its budget proposals released on Tuesday.
The US foots more than a quarter of the UN's $7.9 billion peacekeeping bill.
A spokesman for UN Secretary General Antonio Guterres said the organisation was willing to discuss how peacekeeping could be made more cost-effective.
Mr Trump's budget proposal declares new "attention on the appropriate US share of international spending at the United Nations".
The document does not provide a detailed breakdown, but Reuters news agency reports the drop in funding for the operations could amount to $1bn.
The spokesman said the UN was studying Mr Trump's plan. "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," he said.
The BBC's Nick Bryant at the UN said such a warning from the organisation was unusually blunt.
The US president does not set the budget, but makes recommendations to Congress. It is understood that the UN secretary general has been lobbying Congress members on the importance of international peacekeeping.
The US provides 28.5 percent of the United Nations peacekeeping budget - almost three times as much as the next-highest contributor, China.
While Mr Trump's plan does not include details, the US is known to want to cap its contributions to a maximum of 25 percent.
There are currently 16 active peacekeeping operations, including Syria, South Sudan, and on the border of India and Pakistan.