VIOLENCE in Nigeria's north and centre is worsening. Increasingly deadly attacks on churches by Boko Haram, an extreme Islamist group, are straining fragile relations between Christians and Muslims. Attacking churches is not new for Boko Haram but it has turned its attention to targets in Nigeria's "middle belt" where the two religions mix, often stoking ferocious retaliation. Christian leaders have been warning that the patience of their flocks "will wear out." Fierce fighting erupted in Jos, a mixed city, on July 7th, killing at least 63 people. A day later a Nigerian senator and several other mourners were gunned down during a mass burial. Parts of the city are now under a dusk-to-dawn curfew. Boko Haram released a statement on July 10th saying it was behind the attacks and would continue to kill officials. It threatened that Christians "will not know peace again" until they accept Islam. The prospect of widespread sectarian unrest is growing. Last month Boko Haram attacked three churches in Kaduna, a northern city that had been largely untouched by the insurgency, killing 21 and igniting four days of violence that left another 100-plus dead. Curfews and daily violence persist in Kaduna and other cities far from Boko Haram's heartland in north-eastern Nigeria. President Goodluck Jonathan claimed in March that the menace would be dealt with by June. But heavy-handed military tactics have only boosted the group. Fleeting efforts to negotiate have failed. After criticism flared when he flew on a prearranged trip to Brazil while Kaduna burned, Mr Jonathan felt obliged to sack his national security adviser and his defence minister. Nigeria, he said, needs "new tactics," yet he failed to explain what they might be. As a southern Christian, he has long been urged by outsiders to work with northern leaders. Sure enough, his new security adviser is a northern Muslim. Boko Haram began its insurgency in 2009 in the north-eastern city of Maiduguri with rudimentary bombs and drive-by shootings; men on motorbikes targeted police and clergy in an attempt to carve out a Muslim state. The group rapidly evolved, with its first suicide-bombing hitting the UN headquarters in Abuja, the capital, last August, killing 25 people. Suicide-bombings, barely known in west Africa until last year, are now the most potent weapon in Boko Haram's arsenal. In November more than 100 people were killed in such bombings as well as in gun attacks. The group is thought to comprise several hundred members, though its hard core of planners and ideologues may number as few as 30. The north, where poverty is rising sharply despite strong national economic growth, is an ideal recruiting ground. Security sources say that boys as young as 15 are carrying weapons for Boko Haram in Maiduguri and Damaturu, two of the worst trouble spots. As violence sparks sectarian clashes and criminal gangs hide behind the sect's name, it is not always clear which atrocities are genuinely the work of Boko Haram. The group probably has no overall leader but relies on separate cells in the various regions. It is unclear how closely they co-ordinate their actions. In Kano, the biggest northern city, a string of attacks and kidnappings suggests a measure of joint planning, with the possibility of a link to al-Qaeda in the Islamic Maghreb (AQIM), as the regional version of the late Osama bin Laden's organisation is known. Some Boko Haram people are said to have been trained by AQIM in Mali but there is no sign that non-Nigerian jihadists control the group. Still, General Carter Ham, head of America's Africa Command, says it has links to AQIM and to Somalia's fearsome Islamist militia, the Shabab.