Four persons were killed and food trucks hijacked when Boko Haram guerrillas attacked a United Nations World Food Programme (WFP) aid convoy in Ngala, Borno state in Nigeria.

“WFP can confirm that a convoy escorted by the Nigerian military including WFP-hired trucks was the subject of an attack by armed groups 35 km southwest of Ngala in Borno State on Saturday,” the spokeswoman Jane Howard said in an emailed statement on Sunday.

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“Four people, including the driver of a WFP-hired truck and a driver’s assistant, were killed in the incident,” the statement said, adding that “WFP is working with the authorities to determine the whereabouts of the trucks.”

The spokeswoman declined to comment on whether the driver and assistant were WFP staff, or give details about the other two people killed.

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Attacks on aid workers are relatively rare in the conflict with the Islamist insurgency, compared with assaults on the military and civilians in Nigeria’s northeast.

The years of fighting have spawned what the United Nations calls one of the worst humanitarian crises in the world, with 8.5 million people in need of life-saving assistance.