Secretary-General Antonio Guterres said in a letter to all UN member nations obtained on Friday by The Associated Press that cash for its regular operating budget could run out by July, which could dramatically affect its operations.
"Either all member states honor their obligations to pay in full and on time - or member states must fundamentally overhaul our financial rules to prevent an imminent financial collapse," he said.
While Guterres didn't name any country in the letter, which was reported earlier by Reuters, the financial crisis comes as the US, traditionally the largest donor, has not paid its mandatory dues to the United Nations.
The US now owes $US2.196 billion ($A3.141 billion) to the UN 's regular budget, plus it will owe $US767 million ($A1.1 billion) for this year, according to a UN official, who was not authorised to comment publicly and spoke on condition of anonymity.
The US also owes $US1.8 billion ($A2.6 billion) for the separate budget for the UN 's far-flung peacekeeping operations, and that also will rise.
The country second on the list for not paying dues is Venezuela, which owes $US38 million ($A54 million), the official said.
The country, whose economy was struggling before the US military raid this month that deposed then-President Nicolás Maduro, has already lost its right to vote in the General Assembly for being two years in arrears.
Guterres said the UN ended 2025 with a record $US1.568 billion ($A2.243 billion) in outstanding dues, more than double the amount outstanding at the end of 2024. The UN official said the Trump administration did not pay any dues last year.
Because so much is owed, the UN 's liquidity reserves nearly have been exhausted, Guterres said, and unless payments drastically improve, the UN will not be able to fulfill the $US3.45 billion ($A4.93 billion) regular budget for 2026 approved unanimously in December by the assembly's 193 members.
The secretary-general stressed another major problem he has raised repeatedly: Under UN financial rules, the organisation is required to pay back unspent money from the regular budget to member states - even if it hasn't received that money in payments. He urged UN member nations to change the requirement immediately.
"I cannot overstate the urgency of the situation we now face," he said.
"We cannot execute budgets with uncollected funds, nor return funds we never received."
The US mission to the UN didn't immediately respond to a message seeking comment.
