* U.S.: Plan targets Sheraton hotel near Lagos
* U.S.: "Groups associated with terrorism" behind the plot
* Lagos has not been attacked by Islamist militants before
* Capital Abuja to host World Economic Forum next week (Adds U.S. team to head to Nigeria next week)
ABUJA, May 3 (Reuters) - 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 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 in the city on 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 U.S. citizens to avoid the hotels. The Sheraton brand is owned by Starwood Hotels & Resorts Worldwide Inc .
Lagos is more frequently visited by foreign business people than Abuja, the seat of Nigeria's government, although oil executives often visit the capital to cut deals.
The Abuja bombs, along with the abduction of 200 girls from a school near the Cameroon border, may overshadow the WEF conference which President Goodluck Jonathan's administration has pledged to protect with 6,000 troops.
The United States has offered to help Nigeria, Africa's top oil producer and one of the most vital U.S. strategic allies in Africa, as it searches for the schoolgirls and battles Boko Haram. U.S. Secretary of State John Kerry singled out the group on Saturday during a speech in Ethiopia.
"The kidnapping of hundreds of children by Boko Haram is an unconscionable crime and we will do everything possible to support the Nigerian government to return these young women to their homes and to hold the perpetrators to justice," Kerry said, addressing a forum in the capital Addis Ababa.
A senior U.S. State Department official travelling with Kerry described that support to Nigeria as broad-based and said that it included helping the government improve internal coordination within its security sector.
The official said a U.S. team, including from the U.S. military's Africa Command as well as the State Department, was due to visit Nigeria next week to "consult with them on how we might be helpful" in the fight against the group.
(Reporting by Tim Cocks; Additional reporting by Phil Stewart in Addis Ababa; Editing by Raissa Kasolowsky and Stephen Powell)