More than 1,800 inmates have escaped from a prison in southern Nigeria after it was attacked by heavily armed gunmen on Monday.

The attackers blasted their way into a prison in the town of Owerri in Imo State, engaged guards in a gun battle, and freed inmates, according to a statement by the national corrections authority.

The assailants, who arrived in the town in pickup trucks and buses, also attacked some police and military buildings, officials said. “I can confirm that the Imo State Command of the Nigerian Correctional Service was attacked by unknown gunmen in Owerri,” Imo state corrections service spokesman James Madugba told AFP.

Thirty-five inmates refused to escape while six have returned, according to reports. The banned separatist group, the Indigenous People of Biafra, has been accused of being behind the attack. The group has denied being involved.

President Muhammadu Buhari called the attack an “act of terrorism” carried out by “anarchists”. He urged security forces to capture the attackers and the escaped prisoners.

Imo State is known for being home to several separatist organizations. The attacks in Owerri come less than two weeks after attacks on four police stations, prison vehicles and military checkpoints in southeastern Nigeria killed at least a dozen security officers, DW reported.