A lack of funds has forced the UN World Food Programme (WFP) to stop providing food vouchers for 1.7m Syrian refugees in Jordan, Lebanon, Turkey, Iraq and Egypt.
In a statement, the WFP said that without vouchers, many families will go hungry.
"For refugees already struggling to survive the harsh winter, the consequences of halting this assistance will be devastating."
The UN agency added that it needs $64m (€51m) to support the refugees for the rest of December.
Suspension of the assistance programme comes as many vulnerable Syrian families enter their fourth bleak winter in difficult living conditions after fleeing a homeland devastated by conflict since March 2011.
"This couldn't come at a worse time," said UN High Commissioner for Refugees Antonio Guterres in a statement.
The impact could be particularly devastating in Lebanon, where more than 1.1 million Syrian refugees are scattered across 1,700 communities, according to the UN refugee agency.
Many live in makeshift settlements, sheds, garages and unfinished buildings.
The electronic voucher programme that is in operation has injected about $800m (€640m) into local shops in the countries hosting refugees, and the WFP said it would immediately resume it if new funding arrived.
It is not clear how hungry Syrian refugees might fill the gap left by WFP suspending its voucher programme.
The agency had warned last month it might be forced to impose such a suspension and said it might have to announce a similar suspension in January for people reliant on aid within Syria.
The WFP has already had to cut rations for 4.25m people in Syria.
Meanwhile, the UN refugee agency UNHCR has said a lack of funding has forced it to prioritise as it helps those in need prepare for winter, with preference for people at higher, colder altitudes as well as vulnerable refugees such as newborn babies.
