The weekend attack on a UN World Food Programme (WFP) compound in central Somalia was the fourth "deliberately targeted" incident in two months, according to the agency.
The 16 August attack in Wajid came less than a month after militants raided two UN compounds in Baidoa and Wajid, stealing equipment and vehicles and forcing the closure of some operations.
"This direct, deliberate and sustained attack on aid organizations and aid workers is intolerable," acting UN Resident and Humanitarian Coordinator Graham Farmer said.
Last week, the UN Children’s Fund (UNICEF) issued a warning of increased suffering for malnourished Somali children if humanitarian supplies continue to be destroyed or looted.
"We are worried about the recent destruction and looting of humanitarian aid supplies in certain areas of central and south Somalia," Bastien Vigneau, UNICEF's chief of emergency in Somalia, told IRIN. "If the situation does not improve, we are looking at dramatic consequences for affected acute malnourished children in the next four to six weeks."
The disruption in delivery of aid would put at high risk at least 1.2 million children under-five and 1.4 million women in central-south Somalia.
On 13 August, UNICEF postponed the distribution of hundreds of tonnes of nutritional supplies for more than 85,000 children in central-southern Somalia because of what it termed "increased hostility towards aid organizations".
South-central Somalia has a nutritional demand "above emergency thresholds", Vigneau said. UNICEF and implementing partners were trying to reach at least 150,000 children countrywide suffering from acute malnutrition.
The violence has also disrupted the distribution of anti-malaria bed nets to more than 100,000 women and children.
"[The postponement of aid delivery] will have an adverse impact, especially in the Middle Shabelle region, where we were involved in campaigns against malaria, diarrhoea and other diseases," a local contractor in central-south Somalia for UNICEF, who requested anonymity, told IRIN.
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UNICEF said it was seeking concrete assurances from local authorities that it would beto continue delivering and storing supplies in-country.
"We hope these assurances will be forthcoming very soon so that we can continue our operations at a level that matches the needs of children and women and prevent the deaths that will otherwise certainly occur," Rozanne Chorlton, UNICEF's Representative to Somalia, said in a 13 August statement.
On 17 May, armed men took over UNICEF's compound in the central Somalia town of Jowhar, destroying or looting large volumes of humanitarian supplies and communications equipment. Emergency supplies stored in a partner's warehouse in Jamaame, Lower Juba region, were reportedly taken in early August.
On 20 July, members of the Islamist Al-Shabab militia group, which is fighting the Somali government and controls parts of the central and southern regions, looted equipment and vehicles from the UN compound in Baidoa.
They also raided the UN office in Wajid, 340km northwest of Mogadishu, and later broadcast a message on a local Somali radio station calling for the closure of several UN offices in the country.