bauchi nigeria four people kill unite nations world food programme wfp aid convoy ambush northeast nigeria wfp spokeswoman say sunday late attack region conflict boko haram near ninth year attack aid workers relatively rare conflict islamist insurgency compare assault military civilians nigeria northeast years fight spawn unite nations call one bad humanitarian crises world 8.5 million people need life-saving assistance wfp confirm convoy escort nigerian military include wfp-hired truck subject attack arm group 35 km southwest ngala borno state saturday say email statement four people include driver wfp-hired truck driver assistant kill incident statement say add wfp work authorities determine whereabouts truck spokeswoman decline comment whether driver assistant wfp staff give detail two people kill military spokesman decline comment last year unite nations suspend aid deliveries nigeria northeastern state borno epicenter conflict humanitarian convoy attack leave two aid workers injure last week nigerian government approve release billion country excess oil account government help fight boko haram insurgency despite two-year narrative boko haram defeat sign government military may abandon narrative nigeria long-term plan corral civilians inside fortify garrison towns effectively cede countryside boko haram early month nigeria replace military commander campaign boko haram half year post military source tell reuters come series embarrass attack islamists four dead ambush world food programme convoy nigeria