YENAGOA, Nigeria—Gunmen killed seven Nigerian police officers in a night of attacks in the southern oil hub of Rivers state, police said on Saturday. The attackers opened fire at a checkpoint on Friday evening, then drove on to hit two police stations, the statement said. Officers fired back, killing two of the raiders and injuring some of the others who escaped in a stolen car, police added. The killings came amid deteriorating security in Rivers state—whose capital, Port Harcourt, is the gateway to the oil-rich Delta region—and other parts of Africa’s largest crude exporter. Last month, Rivers State banned people crossing its borders at night in a bid to stop killings of police, customs, civil defense officers, and army soldiers. On Friday, the armed men drove up in two Toyota Hilux vans at 1930 GMT and killed two officers at a checkpoint at Choba Bridge, police spokesman Nnamdi Omoni said …