U.N. spokesman Stephane Dujarric said the secretary-general has written to all members saying "the organization runs the risk of depleting its liquidity reserves by the end of the month and defaulting on payments to staff and vendors."

By the end of September, Dujarric said, member states had paid only 70% of the total assessment for the regular budget, compared with 78% at the same time last year.

According to the U.N., 129 countries had paid $1.99 billion in dues for the U.N.'s 2019 operating budget by Tuesday. It said $1.386 billion is owed for this year.

In addition to the United States, other countries that haven't paid their dues are Brazil, Iran, Israel, Mexico, South Korea, Saudi Arabia and Uruguay.

Because of the U.S. government's budget calendar, Washington usually pays its dues in October.

According to the U.N., the U.S. owes $674 million for the 2019 regular budget and $381 million for previous regular budgets.

The U.S. is also in arrears in payments for the separate budget for the U.N.'s far-flung peacekeeping operations. It owes $255 million for peacekeeping missions that have been closed and $2 billion for active peacekeeping missions, according to the U.N.

Dujarric said measures the U.N. Secretariat put in place early in the year to align expenditures with cash inflows have averted "major disruptions" but "are no longer enough."

Since the U.N. Secretariat could face a default in salaries and payments by the end of November, Dujarric said the secretary-general has requested immediate steps including further reductions in official travel and postponing spending for goods and services.

In addition, Dujarric said, events scheduled outside official meeting hours are being discontinued and conferences and meetings may have to be postponed or their services adjusted.

The secretary-general stressed that cash flow is a recurrent problem and the U.N. is now driven to prioritize work based on cash availability, thus undermining its mandates and obligations "to the people we serve," Dujarric said.

"The secretary-general therefore looks to member states to resolve the structural issues that underlie this annual crisis without further delay," Dujarric said.