Amnesty International on Thursday accused Nigerian security forces of using excessive force and killing at least 115 people in a crackdown on Biafran agitators in the last four months.
Violence flared in Nigeria's south-eastern states this year, claiming the lives of at least 127 police or members of the security services, according to the police.
Some 20 police stations and election commission offices have been attacked.
The Indigenous People of Biafra seeking independence for the Igbo, and its extremist wing Eastern Security Network have been blamed for the violence, but IPOB has denied the charges.
Amnesty said that in response, security forces, including the military, police and the Department of State Services intelligence agency have killed dozens of gunmen, as well as civilians, where attacks have taken place.
'The evidence gathered by Amnesty International paints a damning picture of ruthless excessive force by Nigerian security forces in Imo, Anambra and Abia states,' said Osai Ojigho, the group's Nigeria director.
The global rights watchdog said it 'documented at least 115 persons killed by security forces between March and June 2021.'
Amnesty said relatives of the victims told the rights group that they were not part of the extremist groups who were attacking security agents.
'Many of the victims were deposited at government hospitals in Imo and Abia state,' it said.
Amnesty said it also documented cases of arbitrary arrest, ill-treatment and torture in the restive region.
It said that in May 2021, Imo state government announced the arrest of at least 400 people allegedly linked to the violence.
'Amnesty International's investigation indicates that most of them were randomly picked up in their homes and off the street and had nothing to do with ESN.'
Local and international rights groups have repeatedly accused Nigerian security forces of rights abuses, but they always deny the charges.
Nigeria has recently intensified a crackdown on separatist agitators, including the arrest and trial of their leaders.
Last month, IPOB leader and founder Nnamdi Kanu was detained in Kenya, according to his lawyers, and brought back to Nigeria to face treason charges.
Another separatist leader, Sunday Adeyemo also known as Sunday Igboho, was arrested in neighbouring Republic of Benin as he attempted to board a flight to Germany.
He is currently detained in Cotonou, faced with a charge of entering Benin illegally.
Igboho is accused of calling for a separate homeland for Yoruba people of southwest Nigeria following alleged killings of locals by Fulani herders.
The Nigerian police are yet to respond to the allegations.
'I have not seen the statement. So I cannot respond,' police spokesman Frank Mba told AFP.