TIFE OWOLABI  MORE than 1 800 prisoners are on the run in southeast Nigeria after escaping when heavily armed gunmen attacked their prison using explosives and rocket-propelled grenades, the authorities said. Nigerian police said it believed a banned separatist group, the Indigenous People of Biafra (IPOB), was behind the attack in the city of Owerri, but a spokesman for the group denied involvement. The secessionist movement in the southeast is one of several serious security challenges facing President Muhammadu Buhari, including a decade-long Islamist insurgency in the northeast, a spate of school kidnappings in the northwest and piracy in the…