The World Food Programme has condemned the killing of two of its drivers taking aid to Sudan's Darfur region.
Pro-government militias are still burning villages in Darfur
Such attacks make it difficult to deliver enough food to the area before the rainy season, said WFP Sudan head.
Across the border in Chad, the UN refugee agency has pulled out of five camps after its workers were targeted in violence which left three dead.
More than 2m people have fled their homes in Darfur - some to Chad - during two years of conflict and violence.
At least 180,000 people have already died from hunger and disease, in addition to those killed in the fighting between pro-government Arab militias and Darfur's rebel groups.
The WFP said its drivers were killed in separate incidents by "bandits" on the road between Ed-Daen and Nyala in south Darfur.
"Such attacks only make drivers extremely reluctant to transport food aid in Darfur and are making it very difficult to deliver enough food before the rains," said WFP's Sudan director, Ramiro Lopes da Silva.
In Chad, refugees have staged violent protests after the UNHCR tried to clamp down on them selling aid supplies in local markets and verify refugee numbers.
At least one policeman and two refugees were killed in the clashes, after aid workers were beaten with sticks and stones.
The UNHCR has now pulled out of four of the 12 camps it runs in eastern Chad, for the 200,000 people who have gone there from Darfur.