Gunmen have killed at least five officers in Nigeria's southeastern Imo state, according to police and witnesses, the latest casualties in a wave of attacks targeting security forces in the region. Tensions were high in the state capital Owerri, where gunmen clashed with security forces late on Monday just hours after the attack on the police station in the nearby city of Okigwe. "I can confirm an attack on Okigwe South Area Command Headquarters by yet to be identified gunmen," police spokesman Orlando Ikokwu told AFP late Monday. "Five officers have been killed, while one is yet to be accounted for." Eye witnesses said six were killed in the attack. "Around evening time on Monday, we saw some people in a bus approach the police station, shoot the gate open, and killing all the six officials who tried to resist them," said Festus Okoye. Another resident, Ugochukwu Amaraizu, had a similar account. "People started running for their safety. I think they killed six police officers. I think I saw them leaving with one person from the cell." Dozens of security personnel have been killed and police stations burnt in a wave of assaults by gunmen in the region. In Owerri, gunmen clashed with security forces late on Monday. "There were gunshots... last night, but we are still investigating the matter," Ikwoku said, without confirming any casualties. Three eye witnesses told AFP there was a face-off in Owerri between gunmen and security forces. In Port Harcourt, in nearby Rivers state, five security officers including two soldiers were killed at the weekend. State Governor Nyesom Ezenwo Wike announced that people would be banned from crossing state borders from 8:00 pm to 6:00 am, effective Wednesday until further notice. "In view of what is going on... the government of Rivers state has decided to restrict night movements into and out of the state from the land borders," he said in a statement. A police memo seen by AFP said the suspects of Sunday's attack in Rivers were IPOB separatists. The Indigenous People of Biafra (IPOB), who has not responded to the claim, is an outlawed movement that wants an independent state of Biafra for the Igbo people, indigenous of southeast Nigeria. Separatist calls are a sensitive subject in Nigeria, after a unilateral declaration of independence in 1967 sparked a brutal 30-month civil war. More than one million people, mostly Igbo, were killed in the war. str-lhd/pma/kjl