ABUJA, Nigeria (AP) — Militants enforcing a separatist lockdown in Nigeria’s southeastern region attacked security forces deployed to restore order, killing five soldiers and six civilians during a shootout, the Nigerian military said Friday.

The soldiers were attacked on Thursday at a checkpoint in Abia state’s Aba town where the separatists were enforcing a lockdown to commemorate the short-lived Republic of Biafra which, in 1967, fought and lost a deadly civil war to become independent from Nigeria, defense spokesman Maj. Gen. Edward Buba said in a statement.

The Indigenous People of Biafra (IPOB) separatist group often uses lockdowns to push for the creation of an independent country in the southeast, decades after the war that killed at least one million people. Hundreds have been killed in recent years in such violent lockdowns and other attacks blamed on the group, which claims its secessionist campaign is peaceful.

The Nigerian army had deployed soldiers to enforce peace in Aba town when the militants “sprang a surprise attack” at their security outpost, the defense spokesman said. “Six civilians were (also) killed in the crossfire,” Buba said.

He added that the Nigerian military, overstretched by other security crises in other parts of the country, would not relent in hunting down the perpetrators. “We would bring overwhelming military pressure on the group to ensure their total defeat,” he said.

Beyond their separatist campaign, the IPOB group is also demanding the release of their leader Nnamdi Kanu, who is being prosecuted for charges of treason and terrorism.

Nigeria’s southeast, once among the safest in the country, is now battling violence and deepening poverty as the violent lockdowns take a toll on economic activities in the region.