The embassy advised U.S citizens to be particularly vigilant around church and other places of worship, locations where crowds gather, government facilities and areas frequented by foreigners because of an ongoing threat from Islamic extremists operating mainly in northeast Nigeria, where there is a 2-month-old state of emergency.
Kidnappings for ransom are relatively common in oil-rich Nigeria both of foreigners and wealthy Nigerians.
The 3-year-old son of a state legislator and four Chinese nationals in two different states all have been reported kidnapped this month.
Some cases are not reported because of fears for the safety of the hostages.
Most are released unharmed after ransoms have been paid, though people have been injured and killed if they resist.
Risk analysts Drum Cussac have reported "a noticeable upsurge" in the number of kidnappings in the southwest of Nigeria including Lagos.
"Foreign nationals were targeted in the affluent Lagos areas of Ikoyi, Victoria Island and Epe-Lekki, sparking speculation that organized criminal gangs have renewed their focus on the city's upscale districts given their relatively permissive security environments," the consultancy said.
The latest piracy statistics from the London-based International Maritime Bureau also reported an increase in kidnappings of the crew of hijacked ships off the coast of Nigeria and the rest of the Gulf of Guinea.
