GENEVA — Facing what it described as a severe cash shortfall, the United Nations food aid organization said on Monday that it had been forced to suspend a voucher program that was helping to feed 1.7 million Syrian refugees in neighboring countries.

The suspension by the organization, the World Food Program, was one of the most drastic cutbacks ever by an emergency relief provider in the nearly four-year-old Syrian crisis, raising the prospect of widespread hunger at the onset of winter.

The cutback in aid will affect refugees in Egypt, Iraq, Jordan, Lebanon and Turkey who receive voucher cards from the program, which work like debit cards so users can buy food in local shops. The same mechanism also provides an economic lifeline to communities struggling to cope with the influx of Syrian refugees that has swelled since the conflict began in 2011.

“A suspension of W.F.P. food assistance will endanger the health and safety of these refugees and will potentially cause further tensions, instability and insecurity in the neighboring host countries,” Ertharin Cousin, the organization’s executive director, said in a statement from its headquarters in Rome. “The suspension of W.F.P. food assistance will be disastrous for many already suffering families.”