A deadly raid by Nigerian troops against a Shiite Muslim sect in mid-December was “wholly unjustified”, Human Rights Watch said in a new report Wednesday, as accusations mounted over the incident in the northern city of Zaria.

In its report of the two-day raid, HRW accused the Nigerian Army of killing and quickly burying the bodies of at least 300 Shia Muslims from the Islamic Movement on Nigeria (IMN) sect in mass graves. It said shots were fired at children. Nigerian activists have estimated that up to 1,000 people were killed between Dec. 12 to Dec. 14. The mass burials, carried out without family permission, make the exact toll hard to calculate.

"At best it was a brutal overreaction and at worst it was a planned attack on the minority Shia group," HRW Africa director Daniel Bekele said.

Nigeria's military was responding to what it claims was an assassination attempt by the sect against Army chief Gen. Tukur Burata. It refutes the HRW account, though it has yet to release its own count. HRW said Nigerian troops attacked a mosque, a burial site and the home of IMN leader Sheikh Ibrahim Al Zakzaky. His wife and son are among the dead.

Sheikh Zakzaky is the founder of IMN, a movement that has an estimated 3 million followers in Nigeria and close ties to Iran. Zakzaky, who is is currently in custody, is a devoted follower of Iran's late Ayatollah Khomeini, even choosing to dress like an ayatollah.

News reports on what happened in Zaria seem to point to a disproportionate reaction by Nigerian troops. According to CNN:

The trouble began Saturday, when thousands of members of the Islamic Movement of Nigeria, a pro-Iranian Shia group, held a march in Zaria, in Kaduna State, to mark the birthday of the Prophet Mohammed.

The Nigerian Chief of Army Staff, Lt. Gen. Tukur Yusuf Buratai, and his convoy wound up trapped in the gridlock.

According to some accounts, a metal object was thrown at the general's vehicle by someone in the crowd. Soldiers mistook the sound for gunshots and apparently thought it was part of an attempt to assassinate the general.

At any rate, soldiers in the convoy opened fire, killing a number of people.

The incident comes as President Muhammadu Buhari tries to focus on the fight against Boko Haram, a Sunni Islamist group active in northeast Nigeria. Mr. Buhari has yet to release a public statement on the incident; a spokesman said the killings were a “military affair.”

Iranian President Hassan Rouhani has called Buhari to demand an explanation as thousands of protesters took to the streets in Iran to protest the killing, as well as in areas of northern Nigeria and parts of India. Mr. Rouhani called for an independent investigation, though IMN has already rejected a committee set up by the government, calling it biased.

This incident is not the first clash between IMN and Nigeria's Army. Muslim leaders have warned the Army to tread carefully, citing Boko Haram as an example of what happens when extrajudicial killings by troops lead to greater militancy and spiraling unrest.

"The history of the circumstances that engendered the outbreak of militant insurgency in the past, with cataclysmic consequences that Nigeria is yet to recover from, should not be allowed to repeat itself," said Muhammad Sa'ad Abubakar, the Sultan of Sokoto and leader of Nigeria's Muslims, in a statement Monday.

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"Nigeria has not learned its lessons,"said Shehu Sani, a senator representing Kaduna, the state within which the massacres occurred. "This was how Boko Haram started, with the extrajudicial killing of their leader in 2009. What happened in Zaria was nothing but an act of brutality by the Nigerian military."

Boko Haram's founding leader Mohammed Yusuf was killed in 2009 after a deadly crackdown by the Army.