The chairman of a local community group, Osi Olisa, said the gunmen also shot people on the road before escaping.
No group has claimed the attack in River State, although suspicion has fallen on an armed group believed to be behind kidnappings and killings in the area.
In November, local defence groups and security officials shot dead the group's leader.
Gun violence is common in parts of Nigeria's oil-producing Niger Delta region, where various militant groups take up arms against multinational companies to demand a share of oil proceeds.
