Nigerian soldiers fired on unarmed Shia children without provocation before unjustified raids that killed hundreds of the minority group in the West African country, Human Rights Watch said on Wednesday.
The US-based group disputed the Nigerian military's version that raids from December 12 to 14 on three Shia locations in the northern town of Zaria followed an attempted assassination of the army chief.
Nigeria's military said the raids came after Shia demonstrators tried to block the convoy of General Tukur Buratai.
Human Rights Watch, however, said the army's version "just doesn't stack up".
"It is almost impossible to see how a roadblock by angry young men could justify the killings of hundreds of people," said Daniel Bekele, Africa director of Human Rights Watch.
"At best it was a brutal overreaction and at worst it was a planned attack on the minority Shia group."
READ MORE: Nigeria accused of killing hundreds of Shia Muslims
According to Human Rights Watch, witnesses at the Hussainniyah mosque said dozens of soldiers took up positions by the mosque at around midday on December 12, at least an hour before the army chief of staff was due to pass by.
Without provocation, the soldiers fired on people coming out of the mosque, initially killing an estimated five people and injuring others, including children attending classes at the centre, according to Human Rights Watch (HRW), which said it interviewed many witnesses separately at locations in Kaduna and Zaria, on December 17 and 18.
A 14-year-old girl attending a maths class in the mosque complex said that she was shot as she walked out of the centre with other children, HRW said.
As many as 1,000 people may have been killed, rights activists say, sparking protests in Nigeria's mainly Muslim north that spread to Tehran, the Iranian capital, and New Delhi in India.
Nigerian Muslims wounded in the raids were dying in detention because they were being denied medical care, the Shia Islamic Movement in Nigeria said on Monday.
READ MORE: Nigeria's Shia detainees 'dying' without medical care
Ibrahim Musa, the movement's spokesman, said in a statement that two of its members died in detention on Sunday.
He said at least 40 other wounded members - including founder Ibrahim Zakzaky - were also not receiving medical treatment.
Musa said police handed over the bodies of 12 members wounded in a protest and later detained in Kaduna city on December 15.