At least two suspected armed robbers have been killed after an explosive device they were carrying went off in Nigeria's oil capital, Port Harcourt, police say. A BBC reporter at the scene says the passenger minibus they were travelling in has been completely destroyed. Two more people are in hospital in a critical condition. The police chief said there was no reason to suspect the blast was linked to Islamist militant group Boko Haram. Boko Haram has staged numerous attacks across northern Nigeria but has not targeted the country's oil industry, based in the south. Other militant groups used to carry out frequent attacks on the oil industry in Port Harcourt and the surrounding region but many have now joined a government amnesty and the area has been relatively calm recently. The BBC's Abdul Mohammed Isa, in Port Harcourt, says the explosion happened close to the city centre and there are large crowds of people looking at the scene.