By Alexis Akwagyiram
KADUNA, Nigeria (Reuters) - Blackened walls and piles of rubble are all that is left of the house of a leader of Nigeria's Shi'ite minority after it was burned down by machete-wielding youths in the tense northern city of Kaduna.
A wave of attacks on members of the Islamic Movement of Nigeria (IMN) Shi'ite sect combined with a security crackdown by the authorities is worsening sectarian rivalries in northern Nigeria, where the army is already fighting Boko Haram, a Sunni militant group that has killed thousands.
The violence risks radicalizing the sect, creating another problem for President Muhammadu Buhari as he struggles with an insurgency in the Niger Delta oil region, secession calls in the southeast and Nigeria's first recession in more than 20 years.
The clashes erupted after the Kaduna state government declared the Shi'ite sect unlawful on security grounds, angering its adherents as anyone convicted of being a member could now be imprisoned for up to seven years.
Shi'ites say the ban effectively invited local Sunni youths, who have complained about the sect staging processions in public, to take on the IMN while it marked the holy day of Ashoura mourning last month.
"Young men armed with machetes came looking for IMN people that day, burning houses and chasing away those who hadn't already run away. They came to kill," said Musa Abubakar, a driver who witnessed the clashes in Kaduna's Tudunwada district.
"Those they caught were badly beaten," he said, adding that dozens of men chased away outnumbered police officers. Two Shi'ites were killed elsewhere in Kaduna state in the clashes.
Anger has been rising in the sect's strongholds in the north since the army killed 347 IMN members in the city of Zaria. They were buried in mass graves after clashes in December 2015, according to a judicial inquiry.
Since then Shi'ite leader Sheikh Ibrahim Zakzaky has been imprisoned at an unknown location without being charged. His followers have daubed "Free Zakzaky" on walls in Kaduna.
"We feel repressed," said Abdul Giwa, an IMN spokesman. "We have the freedom of religion. What the government should do is tolerate and understand us."
Security analysts draw parallels with Boko Haram, whose insurgency began in 2009 after security forces killed hundreds of its members and its leader Mohammed Yusuf died in custody. Zakzaky was badly wounded in the December clashes.
Posters calling for Zakzaky's release were quickly torn down, said Giwa who was speaking to the press because the group's spokesman had gone into hiding.
Africa's most populous nation of 180 million combines a predominantly Christian south and mainly Sunni Muslim north.
Shi'ites are estimated at less than 4 million, according to a 2009 report by the U.S.-based Pew Research Center, although there are no official figures. Human Rights Watch estimates that IMN has around 3 million members.
The IMN was founded in the 1980s after the revolution in mainly Shi'ite Iran in 1979, which inspired the sect's founders.
Kaduna officials, made up like Buhari and other northern elites of Sunnis, see the sect as security threat.
"The IMN is more of a political organization than a religious organization," said Kaduna governor Nasir El-Rufai, adding that the IMN was declared an "unlawful society" because it constitutes "a threat to the security and good governance of the state".
"Over the last 30 years they have been engaged in all kind of violations of Nigerian law, from murder to abduction of young children to blocking of highways to forceful acquisition of property of their neighbors," El-Rufai said. "And they also operate a military wing."
He rejected the comparison with Boko Haram as a "simplistic analogy" because the Sunni group's leader was subjected to an extra-judicial execution. In contrast, Kaduna state would bring charges against Zakzaky in court, if he was suspected of wrongdoing after he was released, he added.
Buhari's two spokesmen refused to comment on Zakzaky's detention, saying it was a matter for the justice minister. He did not answer repeated phone calls.
Buhari, a former military ruler, said in August he would study a judicial inquiry report recommending that soldiers who killed hundreds of Shi'ites in December should be put on trial, but no decision has been announced.
A Western diplomat said that Islamic State, the militant group which has seized territory in Syria and Iraq, could use Boko Haram to stir up sectarian strife between Shi'ites and Sunnis in Nigeria.
"Their standard approach is to find religious fault lines and play on them," he said.
Signs of sectarian tension are already emerging in Kaduna, a major city in the north where poverty, corruption and unemployment have helped Boko Haram and hardline Islamists recruit angry youths.
Many IMN members say the police and army, institutions that in the north are dominated by Sunnis, were behind the bloodshed that marred the Ashoura processions.
"The police escorted the thugs," said Giwa, adding that he believed authorities were being influenced by Wahhabism, Saudi Arabia's official ultra-conservative Sunni Muslim school that often labels Shi'ites as heretical.
"We feel they are the ones who might have influenced the government into this action," he said, leaning forward.
Kaduna's police denied any involvement in attacks on IMN or any other Shi'ites in the state. Kaduna state police spokesman Aliyu Usman said "hoodlums taking advantage of the situation" were responsible for the attacks last month.
"We're protecting them. We know what members of the public want to do to them," he said, adding that 11 people had been charged with offences in the wake of the violence.
In the rundown district of Tudunwada, just a few meters from the graffiti calling on Zakzaky's release, local Sunnis had little sympathy for members of the Shi'ite sect.
"Everybody has the right to practice their own religion, but not if you impede other people's rights," said Muhammad Salis, a 32-year-old carpenter.
Abdullahi Ahmed, a 43-year-old civil servant, went further. "They take the law into their own hands," he said, accusing the sect's members of blocking roads during religious gatherings.
(Additional reporting by Garba Muhammad and Felix Onuha, in Abuja; Editing by Ulf Laessing and Giles Elgood)