"It’s murder," Jan Pronk, the UN Secretary-General’s Special Representative for Sudan, told reporters in Khartoum on Wednesday. "It is murderous."

Pronk said he was very concerned about the "increased number of security incidents carried out by bandits", and noted that the African Union (AU) estimated that banditry in Darfur was leading to the death of one person every three days.

"Both incidents happened after dark and we don’t know who is responsible for this. The African Union has been requested to investigate the matter," Julie Stewart, WFP spokesperson, told IRIN on Thursday.

In the first incident, the driver was attacked between Assalaya and Yassin and shot in the head. The truck, with its cargo intact, was recovered the next day when other drivers found it on the road.

In the second incident, a truck was attacked in Assalaya as it returned from Nyala after delivering its load. The driver was shot and killed, but his assistant, although wounded, managed to drive the vehicle back to Ed-Daen.

Stewart added that in a third incident that day, three other trucks were shot at in the Yassin area, but nobody got hurt and the trucks were not stopped.

"WFP strongly condemns these attacks and extends its condolences to the families of the victims. 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," Ramiro Lopes da Silva, WFP’s Country Director in Sudan, said in a statement on Thursday.

"If we allow this status quo of violence and a general climate of lawlessness and insecurity to continue, we simply cannot reach all the people in need," Lopes da Silva added. "People’s lives are being needlessly lost at the very time when they are working to save lives. We need action now to stop this double tragedy."

"This kind of banditry has been going on for months," Stewart said, adding that WFP and the AU had been discussing means to prevent the banditry and killings and secure roads for trucks and convoys carrying food and other aid into the three Darfur states.

On 5 May, a convoy of five WFP-contracted trucks was stopped in North Darfur state between Kafod and Neni while heading toward Kutum. One truck was released but its load of 25 mt of sorghum was looted. A second truck was stolen but was recovered in Neni later that day.

In January, a driver of a WFP-contracted truck was shot and killed in Darfur. In April, another driver was shot and wounded while one other had his hands broken.

A total of 11 WFP-contracted trucks were still missing in Darfur after a series of attacks since November last year, WFP said.

"We estimated in November last year that we would need to be feeding 2.8 million people in Darfur at the height of the rainy season in July and August, but now we are planning for the worst and are looking at how we could get food for up to 3.25 million people in Darfur," Lopes da Silva said.

The war in Darfur pits Sudanese government troops and militias - allegedly allied to the government - against rebels fighting to end what they have called marginalisation and discrimination of the region's inhabitants by the state.