UNITED NATIONS, July 26 (Xinhua) -- The United Nations is urging its member states to pay their membership dues in full and on time as the world body is experiencing an unprecedented cash shortfall, a spokesman said Thursday.

UN Secretary-General Antonio Guterres has written a message to UN staff, outlining his concerns, said Stephane Dujarric, Guterres' spokesman.

The secretary-general said the cash shortfall has never been so serious and so early, and asked for measures to reduce non-staff expenditure.

"Caused primarily by the delayed contributions of member states to the regular budget, this new cash shortfall is unlike those we have experienced previously. Our cash flow has never been this low so early in the calendar year, and the broader trend is also concerning: we are running out of cash sooner and staying in the red longer," Guterres wrote in the message seen by Xinhua.

"I have appealed to member states to pay their assessments on time and in full, and highlighted the risk the current situation poses to the delivery of mandates and to the reputation of our organization. For our part, we will need to take measures to reduce expenses, with a focus on non-staff costs," Guterres wrote.

Dujarric said that only 112 out of the 193 UN member states have paid their dues so far.

He explained that cash shortfalls are not unusual as the United Nations has no control of the timing of payments by member states. But the problem is that the shortfall is coming too much and too soon this year.

"The drought is coming earlier and harder than it has in previous years," he told reporters.

"The United Nations has no flexibility as a government (has) in terms of controlling income and timing of income. We rely on member states to pay their dues in full and on time."

The United States, which contributes 22 percent of the current UN regular budget, has not paid it dues. But Dujarric said the United Nations is not pointing a finger at any individual country as member states have different fiscal years.

The United Nations has to wait for months before it can receive the big chunk of payment from Washington as the U.S. fiscal year starts on Oct. 1.

As to the expenditure cuts mentioned in Guterres' letter to UN staff, Dujarric said: "We are looking at travel and office supplies in terms of cost saving -- things we can immediately control," he said.

The UN regular biennial budget for 2018 and 2019 is about 5.4 billion U.S. dollars. The United States is the largest contributor, followed by Japan, China, Germany, France and Britain. The six countries combined shoulder more than half of the UN regular budget.

The world body has a large, separate peacekeeping budget, with the United States being the largest contributor as well.