Suspected separatist gunmen have killed at least eight police officers over the past three days in southeast Nigeria, authorities said Monday, raising concerns ahead of weekend presidential elections.

Four officers were killed in an attack at a station in Anambra state Monday, while authorities searched for suspects accused of killing four others over the weekend local police spokesman Tochukwu Ikenga said. The assailants opened fire on officers while detonating explosives, he said, adding that three of the attackers were killed and two more arrested.

Police blamed the attacks on a separatist group known as the Indigenous People of Biafra, or IPOB, which wants the southeast region to gain independence from the West African country.

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Authorities have accused the IPOB of instigating violence which has led to many deaths in the conflict-riddled region and stoked fears about the ability of Nigeria’s security forces to protect voters at the polls.

The election commission might not be able to deploy to some polling stations because of security concerns, said Festus Okoye, an official with Nigeria’s Independent National Electoral Commission.

“The security agencies have promised that they have the capacity to secure our communities to make it possible for people to vote,” he said. “(But) for people in zones that are still in conflict, there is absolutely nothing we can do.”