The Islamist militant group, Boko Haram, which is fighting a violent insurgency in northeast Nigeria, has about 4,000-6,000 "hardcore" fighters, US intelligence officials said on Friday.
In an assessment of the group, whose five-year uprising has included massacres and kidnappings and spread from Nigeria into neighboring states, the officials said they did not believe it posed a major threat to Nigeria's oilfields in the south.
The officials, who spoke on condition of anonymity, said the militants were believed to be still holding about 300 schoolgirls they kidnapped early last year and had dispersed them to multiple locations.
Around 10,000 people were killed in Boko Haram attacks last year. The Sunni Muslim group poses the biggest security threat in Nigeria, Africa's top oil producer and biggest economy.