Six military advisers arrived in Nigeria Friday to help locate the 276 schoolgirls kidnapped by Boko Haram, bringing the total number U.S. military personnel there to 18. But a senior defense official told NBC News that Nigeria's military needs to "step up" after having received U.S. training assistance for the past two years. Boko Haram, an Islamic extremist group, has been terrorizing northeast Nigeria for years, destroying churches, kidnapping scores of people and burning down schools.

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