The United Nations World Food Program says two workers delivering food in southern Sudan have been shot and killed.

The agency says on Monday unknown gunmen shot dead a 47-year-old driver behind the wheel of his truck after he delivered humanitarian aid. His assistant was killed while trying to flee the attack, which took place near the town of Mayom.

The deaths bring to five the number of WFP truck drivers killed in Sudan during the past three weeks. On March 22, two truck drivers were stabbed to death near Mayom. On March 25, a driver was killed on the main route to Nyala in the South Darfur region.

The World Food Program says insecurity has delayed its deliveries in Sudan, and reduced by half the amount of food normally transported into Darfur.

The WFP says 56 of its trucks have been hijacked in Sudan since the beginning of the year. Thirty-six trucks and 24 drivers remain missing.

Some information for this report was provided by AFP, AP and Reuters.