The World Food Programme suspended work in parts of northern South Sudan after a river convoy carrying 1,500 tonnes of aid was attacked and looted, it said Wednesday.
There has been rising violence in recent weeks between supporters of President Salva Kiir and his long-time rival Riek Machar -- mostly centred on Jonglei state where at least 280,000 people have been displaced, according to a United Nations report on Tuesday.
WFP said it came under attack last week in neighbouring Upper Nile state, which has also seen major unrest over the past year.
"A 12-boat WFP convoy transporting over 1,500 metric tons of vital food assistance was attacked multiple times by armed youth," it said in a statement.
The cargo, which also included non-food items from partners, was looted overnight "without security intervention" from local authorities, WFP said, despite their safety assurances.
"WFP has suspended all activities in Baliet County until the safety and security of its staff, partners and contractors is assured," it said, and until the government recovers the stolen items.
"Attacks on humanitarians are never acceptable," WFP added.
The commissioner for Baliet county, Joseph Deng, told AFP by phone that government security personnel tried to protect the convoy after one of the 12 boats was initially seized by "armed youth".
"The youth mobilised themselves again and came and attacked the 11 boats and took them all. The security forces that were present were overpowered and couldn't protect the boats because the youth were all armed and shooting," Deng said.
He described the WFP announcement as "unfortunate".
"You can't punish these innocent people because of a few individuals," Deng said.
"We are ensuring this doesn't happen again. We are also increasing the number of security forces to protect all humanitarian supplies," he added.
Last week WFP and Doctors Without Borders (MSF) denounced government restrictions on humanitarian access to Jonglei, with MSF also forced to evacuate one of its health facilities due to armed clashes.
South Sudan, the world's newest country, has been beset by civil war, poverty and massive corruption since it was formed in 2011.
