Kaduna - A newly banned Shiite group in northern Nigeria said on Thursday it was appealing in court against the government decision that observers are warning could spark sustained violence.The Islamic Movement in Nigeria said the ban by Kaduna state officials violates Nigerians' rights to religious freedom. The ban, announced earlier this month, was made official with its publication on Wednesday."We are already seeking redress in the court and will pursue the case to its logical conclusion," the group's statement said.Nigeria is almost equally divided between Christians and Muslims, most of them Sunni. Tensions with the Shiite population are high after the army gunned down more than 300 people in an attack in December on the Islamic Movement in Nigeria's headquarters.The government has said the group provoked the attack, with the military accusing it of trying to assassinate the country's army chief — a claim that human rights groups have called unbelievable.Full blown insurgency The group's leader, Ibraheem Zakzaky, has been in custody since December.On October 12, the group said 13 of its members had been killed when security forces and "paid thugs" attacked religious gatherings in multiple locations in northern Nigeria. Officials did not confirm the death toll.Earlier this week, the group said several dozen women and children members had been detained.Kaduna officials say the ban applies only to the group and not Shiites in general. But Human Rights Watch says the ban "appears to have triggered" anti-Shiite violence.The Nigerian risk analysis firm SBM Intelligence went further this week, warning in a report that a "reckless" crackdown could result in a "full-blown insurgency" — just as it did with the Boko Haram extremist group, which is now blamed for more than 20 000 deaths over the past several years.