Nigeria on Wednesday faced calls to prosecute soldiers for using excessive force against Shiite Muslim protesters, exactly three years after a military crackdown killed more than 300.Human Rights Watch said the security forces had shown a "pattern of brutality" towards the Islamic Movement of Nigeria (IMN) and warned such action could worsen national security.More than 40 people were killed in three days of protests in the capital, Abuja, in October when the security forces fired live bullets at crowds. The army maintained six people died and that soldiers acted in self-defence because protesters were trying to seize weapons and ammunition, and that troops came under attack.Aniete Ewang, Nigeria researcher at Human Rights Watch, said: "The repression against the IMN Shia Muslim group by government security forces risks creating grievances that could worsen Nigeria's already precarious security situation."The increasing spate of protests by the group is a cry for justice that authorities would do well to heed."She added: "Any unlawful use of violent force against processions and protesters is highly likely to be counterproductive as well as a crime."Mass grave HRW called on the authorities to "end impunity" for the attacks, investigate crackdowns on other protests and "hold anyone found responsible for using unlawful force to account".The IMN and its leader Ibrahim Zakzaky have been at loggerheads with the government for years because of his call for an Iranian-style Islamic revolution in Nigeria.Nigeria is almost evenly split between a mainly Sunni Muslim north and a largely Christian south.On December 12, 2015, soldiers killed 347 IMN members in a three-day crackdown in the northern city of Zaria, after accusing them of blocking the road and threatening the chief of army staff.The bodies were buried in a mass grave, the IMN headquarters was razed while Zakzaky, his wife and hundreds of others were arrested and charged.Scores have since been acquitted because of lack of evidence while several rulings have ordered the release on bail of Zakzaky, who lost the sight in one eye during the clashes.But the government has repeatedly refused to obey the courts and been warned its targeting of the IMN risked radicalising the group in the same way as Boko Haram.The Sunni jihadist group's insurgency in northeast Nigeria has killed more than 27 000 people since 2009, left some 1.8 million others homeless and caused a humanitarian crisis.Yet President Muhammadu Buhari has accused Zakzaky of committing "atrocities", despite him not having been tried or convicted, in comments that raised concerns about due process.The IMN has since 2015 accused the authorities of using excessive force to curb their activities across the country and that at least 110 people have been killed.