Kano - Nearly 350 bodies were buried in a mass grave in northern Nigeria after clashes between the army and supporters of a Shi'ite cleric, a public official has told an inquiry into the unrest.The testimony on Monday from Muhammad Namadi Musa, the director-general of the Kaduna State religious affairs office, lends weight to claims that at least 300 people were killed in the violence in December last year.Amnesty International, which has previously said "hundreds" were killed, said the revelation was "an important first step in bringing all those suspected of criminal responsibility" to trial."It is now imperative that the mass grave sites are protected in order that a full independent forensic investigation can begin," said the organisation's Nigeria director, MK Ibrahim."The bodies must be exhumed and Nigerian authorities should immediately reveal the whereabouts of those held in unacknowledged detention and either charge or release them."The clashes came after the army said supporters of the cleric Ibrahim Zakzaky, who heads the pro-Iranian religious sect Islamic Movement of Nigeria (IMN) group, tried to kill the chief of army staff.Zakzaky has not been seen since his home and the IMN mosque in Zaria, Kaduna state, were destroyed, prompting calls for his release and criticism that the government is flouting due process.Musa said told the hearing he was ordered on December 13 to travel to Zaria with the Kaduna state commissioner of police "to find out the number of corpses and how they would be buried".At the Ahmadu Bello University Teaching Hospital (ABUTH) "we counted 156 corpses", while 191 others were collected from the army base in Zaria, he said.Army denials "Most corpses were covered with black materials and they included women and children," he told the inquiry, saying the bodies were transported for burial in a convoy of trucks with military escort.Earlier, the secretary to the Kaduna state government, Lawal Balarabe Abbas, said the mass burial was authorised by "a warrant obtained from a chief magistrates' court in Kaduna".Nigeria's military, which has been accused of human rights abuses against civilians in the insurgency by Sunni Muslim jihadists Boko Haram, has said its troops acted appropriately.Chief of Army Staff General Tukur Yusuf Buratai in January told a separate inquiry by the National Human Rights Commission that soldiers "acted in accordance to the rule of engagement" and their orders.No official death toll has been released but Human Rights Watch has said "at least 300" were killed. The Nigerian army has said the high death toll numbers were "unsubstantiated". One medic at ABUTH told AFP in January he counted at least 400 bodies in the morgue on the evening of December 12 while locals said as many bodies also littered on the streets.The IMN has said some 730 members were unaccounted for, "either killed by the army or... in detention". In February, prosecutors said 191 IMN supporters in custody were charged with firearms and public order offences.Zakzaky and the IMN have previously clashed with Nigeria's secular authorities over their quest to establish an Islamic state through an Iranian-style revolution. The cleric has periodically been incarcerated for alleged incitement and subversion.