Amnesty International on Friday accused Nigeria's military of deliberately shooting dead more than 350 Shiite Muslims, burying them in mass graves and destroying evidence of the crime.

The rights group also dismissed as baseless army claims the protesters from the Islamic Movement of Nigeria (IMN) group wanted to kill the head of the army before the clashes last December.

The military, which has been repeatedly accused of abuses against civilians in its fight against Sunni Muslim jihadists Boko Haram, has maintained its troops acted appropriately.

There have been fears the military action against the Shiite group in Zaria could trigger another violent uprising similar to that of Boko Haram, whose insurgency has left some 20,000 dead since 2009.

Amnesty's report -- "Unearthing the truth: unlawful killings and mass cover-up in Zaria" -- comes in the wake of conflicting claims about the violence in the northern city.

- Evidence destroyed -

Two days of violence began on December 12 when IMN supporters of the pro-Iranian cleric Ibrahim Zakzaky attending a religious ceremony refused to allow the chief of army staff's convoy to pass.

Amnesty said its investigations indicated the military acted "unlawfully" by shooting "indiscriminately" at unarmed protestors.

"It is not clear why the army launched such a 'military operation' in response to a law and order situation," the report said.

"The Nigerian army has provided no evidence to substantiate its claim that IMN protesters attempted to assassinate the Chief of Army Staff."

It added: "The Nigerian military burned people alive, razed buildings and dumped victims' bodies in mass graves."

Most of the evidence was "meticulously destroyed", Amnesty said, accusing soldiers of trying to cover up the carnage by limiting access to conflict sites.

"Bodies were taken away, sites were razed to the ground, the rubble removed, bloodstains washed off, and bullets and spent cartridges removed from the streets."

Soldiers sealed the areas around Zakzaky's compound, which was destroyed with a cultural centre, and the site of the street protests for several days.

Medical personnel at the Ahmadu Bello University Teaching Hospital said the military cordoned off the area around the morgue for two days.

- Mass grave -

A Nigerian public official said last week that 347 people, including women and children, were buried in a mass grave under military supervision and authorised with a court warrant.

The testimony came at one of several public inquiries set up to investigate the circumstances of the violence, which has led to scores of IMN supporters being charged.

Nigerian media on Thursday said prosecutors in Kaduna were seeking the death penalty for 50 IMN members for killing a soldier in the initial incident in Zaria.

Amnesty provided satellite images purporting to show the location of a "possible mass grave" in the Mando area near the state capital, Kaduna city, approximately 80 kilometres (50 miles) away.

Despite President Muhammadu Buhari's pledge to investigate evidence of war crimes, "to date no concrete steps have been taken to end endemic impunity for such crimes", Amnesty said.

Zakzaky, who lost one eye and was partly paralysed in the violence, has been held incommunicado since December without access to his lawyers until recently.

He has previously fallen foul of Nigeria's secular authorities and been imprisoned for calling for an Iranian-style revolution to create an Islamic state in the north.