From the U. S. State Department:

The Department of State warns U.S. citizens of the risks of travel to Nigeria, and continues to recommend that U.S. citizens avoid all but essential travel to the following states because of the risk of kidnappings, robberies, and other armed attacks: Bayelsa, Delta, Edo, Plateau, Gombe, Yobe, Bauchi, Borno, and Kano states. The Department also warns against travel to the Gulf of Guinea because of the threat of piracy. Violent crime remains a problem throughout the country and is perpetrated by both individuals and gangs, as well as by persons wearing police and military uniforms. Based on safety and security risk assessments, travel by U.S. officials to all northern Nigerian states (in addition to those listed above) must receive advance clearance by the U.S. Mission as being mission-essential. U.S. citizens should be aware that in light of the continuing violence, extremists may expand their operations beyond northern Nigeria to the country’s southern states. This Travel Warning replaces the Travel Warning for Nigeria dated February 29, 2012, to update information on the continued violent activities in the country. ...

U.S. State Department Warning