The UN says it has stopped distributing food in Somalia's capital, Mogadishu, after government troops detained the local head of the World Food Programme.
Many people displaced by this year's fighting depend on food aid
The WFP says about 60 soldiers stormed the UN compound and no explanation has been given for Idris Osman's detention.
UN Secretary General Ban Ki-moon has strongly condemned the troops' "forceful and illegal" actions.
Correspondents say the arrest may be linked to the power struggle between the president and his prime minister.
The mayor of Mogadishu, who gave the WFP permission to distribute food to 75,000 people in the city through a network of mosques, is close to Prime Minister Ali Mohammed Ghedi.
The security services that arrested Mr Osman are the president's men.
Unrest since the ousting of Islamists by Ethiopian-backed troops in December has displaced thousands of people in and around the capital.
The BBC's Africa editor Martin Plaut says that because control of food aid is a key weapon in winning popular support, whoever is seen to control the aid is in a powerful position.
The WFP said Mr Osman was being held by security services in a cell close to the presidential palace, adding that the detention violated international law.
Mr Yusuf (l) and his prime minister (r) no longer see eye to eye
Mr Ban pointed out that the Somali government had signed up to the 1946 Convention on Privileges and Immunity in January 2006.
The suspension of food deliveries to more than 75,000 people in the capital comes just two days after they were restarted for the first time since June.
Correspondents say there is a history of tension between the interim government and the WFP, which is the biggest UN agency operating in the country, providing emergency food relief to more than two million Somalis.
On Tuesday, Mogadishu witnessed some of the worst fighting since the Union of Islamic Courts, who ruled much of Somalia for six months last year, were driven from power.
Meanwhile in Baidoa, where parliament sits, the speaker halted a debate where MPs were considering the future of the government.
President Abdullahi Yusuf wants parliament to sack Prime Minister Ghedi as he says his term has expired according to the federal charter.
But the speaker of parliament told MPs that the African Union and Ethiopia have asked for the debate to be stopped as Mr Ghedi has been called to Addis Ababa to try to resolve the rift with President Yusuf.
Both Mr Ghedi and Mr Yusuf ascended to power with the backing of Ethiopia but have fallen out over reports that they favoured rival concerns interested in oil exploration contracts.
Despite having international support, diplomats argue that the transitional government has failed to set up institutions to reconstruct the country.
The UN's special envoy for Somalia, Ahmedou Ould Abdallah, has warned that a vote of no confidence may derail the peace process.