BAUCHI, Nigeria (Reuters) - Four people were killed when a United Nations World Food Programme (WFP) aid convoy was ambushed in northeast Nigeria, a WFP spokeswoman said on Sunday, in the latest attack in the region as the conflict with Boko Haram nears its ninth year.

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.

“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,” she said in an emailed statement.

“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.

A military spokesman declined to comment.

Last year, the United Nations suspended aid deliveries in Nigeria’s northeastern state of Borno, the epicenter of the conflict, after a humanitarian convoy was attacked, leaving two aid workers injured.

Last week, the Nigerian government approved the release of $1 billion from the country’s excess oil account to the government to help fight the Boko Haram insurgency, despite a two-year narrative that Boko Haram has been all but defeated.

There are other signs the government and military may be abandoning that narrative.

Nigeria’s long-term plan is now to corral civilians inside fortified garrison towns - effectively ceding the countryside to Boko Haram.

Earlier this month, Nigeria replaced the military commander of the campaign against Boko Haram after half a year in the post. Military sources told Reuters that came after a series of “embarrassing” attacks by the Islamists.