Lagos - Islamist sect Boko Haram and Nigerian security forces might well have committed crimes against humanity during three years of conflict that has killed at least 2 800 people, Human Rights Watch (HRW) said on Thursday.
Crimes against humanity are offences that can lead to prosecution by the International Criminal Court (ICC).
Boko Haram says it is fighting to create an Islamic state in Nigeria, and its fighters have killed hundreds in bomb and gun attacks since launching an uprising in 2009. The sect has become the No. 1 security threat to Africa's top energy producer.
The report documents multiple cases of abuses by Islamists, including brutal killings of Christian civilians and the assassination of Muslim clerics who criticise them.
Some of these attacks were "deliberate acts leading to population 'cleansing' based on religion or ethnicity". The ICC defines crimes against humanity as grave offences that are "widespread or systematic".
There was no immediate reaction from Boko Haram.
The report also accused Nigeria's joint military and police joint taskforce (JTF) of "physical abuse, secret detentions, extortion, burning of houses, stealing money during raids, and extrajudicial killings of suspects".
"Despite allegations of widespread security force abuses, the Nigerian authorities have rarely held anyone accountable ... further solidifying the culture of impunity for violence."
The study came as Nigeria's military tried to fend off accusations of a shooting spree in the insurgent stronghold of Maiduguri on Monday that residents say killed at least 30 civilians.
Asked about the report, Lieutenant Colonel Sagir Musa, JTF spokesman for Borno state, of which Maiduguri is the capital, reiterated a statement on Wednesday that there was no evidence of such abuses.
"There is no established or recorded case of extrajudicial killing, torture, arson or arbitrary arrest by the JTF in Borno state", where most of the violence has occurred, he said.
"It is important to state that terrorists killed were during gun battles with the JTF troops", not executions, he said.
The military campaign against Boko Haram has had some success - limiting Boko Haram's ability to carry out large scale attacks, but the heavy-handedness has angered locals.
"These killings, and clashes with the group, have raised the death toll of those killed by Boko Haram or security forces to more than 2 800 people since 2009," the HRW report said.