ABUJA - The United States has warned its citizens of a plan to attack one of two Sheraton hotels near Lagos, Nigeria's main commercial hub which attracts many foreign business people and which so far has been spared by the country's violent Islamist militants.In a statement on its website, the State Department said those behind the plot were "groups with 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 (WEF) for Africa that will be held there between May 7-9.Islamist militant group Boko Haram, which is fighting to carve an Islamic state out of religiously mixed Nigeria, is still mostly confined to the northeast, although it has struck across the north and in Nigeria's capital, in the centre.The group has never attacked Lagos, a sprawling city of 21 million people, although its leader Abubakar Shekau has threatened to do so."As of late April, groups associated with terrorism allegedly planned to mount an unspecified attack against the Sheraton Hotel in Nigeria, near the city of Lagos," the State Department said late on Friday."There was no further information regarding which of the two Sheraton Hotels in Lagos was the possible target ... There is no further information regarding the timing or method of attack."It cautioned US citizens to avoid the hotels. The Sheraton brand is owned by Starwood Hotels & Resorts Worldwide Inc.