The United States has warned its citizens of a plan to attack one of two Sheraton hotels near the city of Lagos.

In a statement on its website, the State Department said those behind the plot were "groups associated with terrorism", but gave no further details. Nigeria is grappling with an increasingly violent Islamist insurgency that has killed thousands in the past five years. Two recent bomb attacks on the edge of the capital Abuja have heightened security concerns ahead of the World Economic Forum for Africa that will be held there between May 7-9.