The World Food Programme (WFP) suspended food aid to more than 1.7 million Syrian refugees in neighbouring countries, blaming a financing crisis caused by unhonoured cash pledges.
The Rome-based UN agency said refugees in Jordan, Lebanon, Turkey, Iraq and Egypt risked going hungry this winter if donors do not urgently provide the $64 million (51 million euros) needed to finance the distribution of food vouchers through December.
"This couldn't come at a worse time," UN High Commissioner for Refugees Antonio Guterres said in a statement. "I urgently appeal to the international community –- support WFP now, don't let refugees go hungry."
While WFP didn't name which countries haven't made good on their commitments, foreign ministers from Germany, Finland and Sweden told reporters in Copenhagen their countries could do more to fill the funding gap.
"We have to strengthen our engagement and give humanitarian aid for the refugees and strengthen the structure of those countries who are hosting the refugees," German Foreign Minister Frank-Walter Steinmeier said.
WFP said the refugees affected by the suspension of food aid included many children in Lebanon and Jordan facing harsh winters without adequate clothing or footwear, and living in tents already caked in mud that has made hygiene precarious.
Most in peril are the tens of thousands of families that are entirely dependant on international food aid, Guterres added.
Distribution of electronic food vouchers is to resume as soon as the pledged cash comes in.
The United States, which has stumped up more than $3 billion for the Syrian people including some $935 million for the WFP since the start of the conflict, also voiced concern.
Washington was urging governments "to do more," said State Department spokeswoman Jen Psaki.
"Without additional contributions, the World Food Programme could be forced to reduce rations for Syrians throughout the region," she warned.
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WFP says it has fed millions of displaced people inside Syria and up to 1.8 million refugees in Lebanon, Jordan, Turkey, Iraq and Egypt in the three and a half years since the conflict erupted.
"A suspension of WFP food assistance will endanger the health and safety of these refugees and will potentially cause further tensions, instability and insecurity in the neighbouring host countries," said WFP Executive Director Ertharin Cousin.
He added: "The suspension of WFP food assistance will be disastrous for many already suffering families."
- No end in sight -
More than half of Syria's population has been forced to flee their homes since war began in their country in March 2011.
Some 3.2 million have fled beyond the country's borders, and more than 7.2 million have become internally displaced, according to the United Nations.
There is no end in sight to the fighting in the brutal conflict that has killed nearly 200,000 people.
Regime troops and jihadists are still battling for control of the country, where foreign fighters have flocked to join the Islamic State group and rival Al-Qaeda-linked militants.
Before the conflict began, Syria had a higher gross domestic product (GDP) than countries such as Tunisia and Jordan, and it ranked favourably on human development indicators including health and education.
But with the brutal violence that followed a crackdown on anti-government protests, regional investors have fled, key infrastructure has been destroyed and the economy has withered.