Civilian vigilantes 'foiled' a suicide attack on the outskirts of Nigeria's flashpoint city of Maiduguri, and killed the two teenage bombers, both girls, police said on Sunday.Boko Haram's militant insurgency has frequently used young women and girls to stage suicide bombings."Two female suicide bombers, about 18 years of age, were sighted by the civilian JTF [vigilantes] and consequently shot dead by security personnel," police spokesman Victor Isuku said in a statement.The would-be bombers were killed Saturday as they tried to break into Maiduguri, the provincial capital of Borno state where Boko Haram was born.Isuku said no one else was killed or injured in the incident, though a rescue services official said a vigilante had been hurt.Abdulkadir Ibrahim of the National Emergency Management Agency (NEMA) said one of the bombers was pregnant.In January, a female bomber with a baby strapped to her back used the infant as a decoy before detonating her explosives, killing several people.Boko Haram has regularly used suicide bombers in their eight-year insurgency, which seeks to establish a hardline Islamist state.A regional military force has pushed the insurgents back from captured territories since 2015, but attacks on soft targets in the region continue.The Boko Haram violence has claimed at least 20,000 lives and forced some 2.6 million others to flee their homes, triggering a dire humanitarian crisis in the region.