The World Food Programme said Monday insecurity in Syria was cutting off half a million people from the food aid they need, while a lack of funds has also forced the UN food agency to slash rations.
Three years into Syria's bloody civil war, the WFP said in recent weeks it had reached a number of areas in the governorates of Damascus, Homs, Raqqa and Daraa that had long remained inaccessible.
But while providing food to some 71,500 people who had not received assistance in months, widespread insecurity had prevented deliveries to another 500,000 people in desperate need, the WFP said in a report.
"On-off convoys into besieged areas can provide temporary relief but WFP still needs proper and sustained access to people to provide life-saving assistance and also to assess the scale of the needs," said WFP deputy chief Amir Abdulla.
The agency aims to reach some 4.25 million people inside Syria each month but in February it said it managed to get food rations to just 3.7 million, as well as to 1.5 million refugees in neighbouring countries.
Lacking funding was also a major challenge, WFP said, explaining that it was forced to cut the size of this month's food basket for vulnerable families inside Syria by 20 percent.
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"As a result, families are receiving fewer nutrients than they require to stay healthy," the food agency said in a statement.
Each person was now receiving a daily ration of 1,530 calories, compared with the 1,920 calories the agency had originally planned, and well below the 2,100-calorie intake recommended by doctors.
"It would be tragic to secure more access in Syria but to then find ourselves in a situation where we do not have the required funds to assist hungry people who have long been under siege," WFP regional coordinator for the Syria crisis Muhannad Hadi said in the statement.
The agency has appealed for $2.0 billion to feed some seven million Syrians displaced in their country or who have fled to neighbouring countries.
The WFP said it spends $40 million a week on its aid operation in Syria and urgently needs $309 million to cover the food needs of vulnerable Syrians until the end of May.
Abdullah told reporters he was hopeful that a March 18 conference following up on a donors' meeting in Kuwait in January would help provide additional funds to WFP and other humanitarian agencies working in Syria.
But if WFP does not receive the required funds, it could be forced to reduce rations by up to 50 percent in April and May, he cautioned.