LAGOS, Nigeria: The United States ambassador to Nigeria has said Islamic extremists continue to move freely from the nation to northern Mali, despite the ongoing French military operation there.

U.S. Ambassador Terence McCulley told journalists on a telephone conference call Thursday that officials "have seen reports for years" about fighters from the radical Islamic extremist network Boko Haram traveling to the region. Boko Haram is the main extremist group fighting Nigeria's weak central government in a guerrilla campaign of shootings and bombings.

McCulley said: "I do believe the Nigerians feel that there is a link between extremist activity in the Sahel and their internal extremist insurgency."

McCulley declined, however, to discuss the U.S. plans to operate a drone base from neighboring Niger, which has worried some in Nigeria's predominantly Muslim north.