The Nigerian army shot dead anywhere from 30 to 40 young men in a raid in the Boko Haram stronghold Maiduguri.
After Reuters obtained a video in which Nigerian troops reportedly shot dead unarmed captives, the government claimed the footage was fake.
Gunmen attacked police HQ in Abuja, the nation's capital, and the Islamist militant group Boko Haram is suspected.
The US is 'concerned' by allegations of human rights violations by Nigerian security forces, Secretary of State John Kerry said.
About 120 Boko Haram Islamists were arrested in Nigeria's northeastern city of Maiduguri on Monday as they were organizing the burial of a commander.
Nigeria said Tuesday it would release several suspects being held for "terrorist activities," including all women in custody.
Female student stands in a burnt classroom at Maiduguri Experimental School, a private nursery, primary and secondary school burnt by the Islamist group Boko Haram to keep children away from school in Maiduguri, northeastern Nigeria on May 12, 2012.