More than 100,000 Somali children depend on WFP food aid
The World Food Programme says US curbs are in part behind its move to shut its Somali feeding programmes for more than 100,000 acutely malnourished children.
The US restrictions affect funding for areas controlled by groups designated as terrorist.
Washington has imposed sanctions on the hardline Somali Islamist group, al-Shabab.
But the WFP says it is doing all it can to get the aid through without it being controlled by the Somali insurgents.
Drought and war has left 109,000 children-under-five acutely dependent on the feeding centres run by the WFP.
But money has run out. The centres are closing, to focus the remaining resources on the most needy - babies under two years old, who would never recover from acute malnutrition.
Aid workers have told the BBC the cuts are the result of a freeze on funding by the United States Office of Foreign Disaster Assistance.
Josette Sheeran, executive director of the WFP, said she was unaware of a ban.
She continued: "We've heard of needing to deal with the particular restrictions they have on where aid goes and needing to look at whether or not we can work in compliance with those restrictions."
The US embargo is stopping American aid funds from reaching the vast areas of southern and central Somalia, where the UN estimates half the population is now in need of food aid.
Ms Sheeran said that "getting help to them inevitably involves dealing with al-Shabab and other hardline groups now in control of the towns and villages across the region".
"We take all precautions to ensure that our food only goes to the most needy and is not handled by any particular political groups in Somalia or elsewhere and in particular al-Shabab in Somalia," she said.
"We hope to work through these difficulties and challenges.
"Right now in Somalia, WFP has by far the biggest programme and there are very few aid groups left functioning there."
Ms Sheeran says the WFP is working with the administration of US President Barack Obama on an almost weekly basis to try to resolve these difficulties.
In the meantime the children of Somalia are going without the food they so badly need.