Recent attacks on trucks contracted by the UN World Food Programme (WFP) to transport food to Darfur and Southern Sudan have slowed down deliveries and affected the UN agency's ability to support returnees, the agency warned.

"Because of truck hijackings, we are moving about half the amount of food that we should be moving into Darfur to pre-position ahead of the rainy season," Peter Smerdon, WFP spokesman in Nairobi, told IRIN. "If this continues in the South we might start having the same problem."

In the latest incident, Hamid Dafaalla, a 47-year-old driver of a WFP-contracted truck, and his assistant were killed in Southern Sudan as they returned from delivering food to Rumbek.

His death brought to five the number of people killed in attacks on WFP humanitarian transporters in Sudan in less than three weeks.

"We are shocked and saddened by this heartless killing," Ebenezer Tagoe, WFP Sudan deputy director, said in a statement. "Attacks against vehicles delivering humanitarian assistance are completely unacceptable."

The attack on Dafaalla, WFP said, occurred 6km from Mayom town in Unity State. The spot is near where two WFP-contracted drivers were stabbed to death on 22 March. Two days later, another WFP-contracted driver was shot dead and his assistant injured while delivering food to Nyala in South Darfur.

"The continued insecurity on the roads in areas where we operate presents not only a serious threat to the drivers, but also to vulnerable people who depend on this food for their survival," Tagoe added.

According to the office of the UN Resident and Humanitarian Coordinator for Sudan, nearly 90 people working with humanitarian operations in Darfur have been abducted, mostly during hijacking incidents, since the beginning of 2008.

This is in addition to killings in the south where about two million people have returned since a peace accord ended more than two decades of conflict in 2005, and rely on aid for survival.

"If security on the roads to Darfur does not improve, WFP risks having to reduce rations in some areas where the flow of deliveries cannot be maintained," Smerdon added.