Militant group Boko Haram aims to set up an Islamic state in Nigeria, a country divided along Muslim and Christian lines.
More than a dozen people were arrested last week on suspicion of allying with Boko Haram in the mixed-religion city of Mubi. This week, at least 20 people, mostly students, were killed by unknown gunmen in the city.
Human Rights Watch, in a 98-page report published Thursday, said violence attributed to Boko Haram likely amounted to crimes against humanity. Government forces, meanwhile, are suspected of committed widespread abuses, including extrajudicial killings, during their campaign against the militant group.
Daniel Bekele, director of African programs at Human Rights Watch, said both sides need to exercise restraint.
"Both sides need to halt this downward spiral," he said in a statement. "Nigeria's government should swiftly bring to justice the Boko Haram members and security agents who have committed these serious crimes."
Human Rights Watch based its research on field studies conducted from July 2010-July 2012. The organization said more than 250 attacks by Boko Haram during the first nine months of this year have left more than 800 people dead.