Hundreds of thousands of displaced Somalis rely on food aid from the WFP
Islamist militants in Somalia are stopping convoys of food reaching more than 360,000 displaced people, the UN's World Food Programme (WFP) says.
The agency says trucks travelling from the capital Mogadishu to camps in Afgoye have been stopped by armed men.
Insurgent group al-Shabaab says the WFP is ruining local farming by forcing Somalis to rely on imports.
But the UN says that without help, Somali farmers cannot supply enough food for those in need.
The camps in Afgoye, just west of the capital, have the largest concentration of refugees in the whole of Somalia.
In January the WFP pulled out of large parts of southern Somalia because of threats from rebel groups.
Al-Shabaab has said any Somalis who co-operate with the WFP would be treated as contributing to the destruction of the Somali economy.
The militants say agricultural productivity has increased in areas they control, but there is no way to independently verify this claim.
They accuse the WFP of distributing expired food which is a public health hazard and say the agency's work is cover for a political agenda.
The agency's Peter Smerdon told the BBC: "The WFP is extremely concerned about the health of the displaced families who rely on humanitarian assistance reaching them.
"The people in Afgoye last received a general food distribution from WFP in November 2009, so we fear that they are going very hungry.
"We fear they are suffering even more because food assistance cannot reach them, and some of them may be forced to leave Afgoye as conditions in the camp deteriorate."
Somalia has been in turmoil since 1991 when its central government collapsed.
The transitional government, helped by an African Union peacekeeping force, runs only parts of Mogadishu and much of southern Somalia is controlled by al-Shabaab.