Two workers delivering food aid to south Sudan have been killed in the third fatal attack on World Food Programme trucks in three weeks, the U.N. organisation said on Thursday.

Humanitarian agencies in Sudan help millions of Sudanese in the south recovering from decades of civil war and in western Darfur, where an ongoing rebellion has left more than 4 million dependent on aid.

But bandits have hindered operations in Darfur and violence has flared recently on Sudan's north-south border.

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

He said the semi-autonomous southern government was investigating the attack.

WFP said driver Hamid Dafaalla was shot dead in his truck on Monday and his assistant killed when trying to flee. The attack was near the location of another attack on March 22 when two other WFP drivers were fatally stabbed.