KADUNA, Nigeria - Multiple church bombings in Nigeria and subsequent rioting by Christian youths targeting Muslims killed at least 21 people on Sunday, officials and residents said.Bomb blasts struck three churches in the northern Kaduna state, the latest in a string of Sunday attacks that has threatened to ignite wider sectarian strife across the religiously-divided country. There was no immediate claim of responsibility for the bloodshed but the attacks were likely to be blamed on Boko Haram.Two churches were attacked in the city of Zaria and one in Kaduna City, killing at least 16 people, according to a police statement. Explosions were also reported in Nassarawa and Barnawa.After news of the blasts spread, Christian youths took to the streets of the main motorway that connects Kaduna City to Nigeria's capital Abuja, attacking motorists who looked Muslim. An AFP reporter said he saw the bodies of 10 people killed by the rioters being taken to the morgue."The official figures from medical personnel on the total deaths from the bombings and protests are 21 deaths and 101 injured," said an official from the National Emergency Management Agency who requested anonymity. Kaduna state authorities immediately imposed a 24-hour curfew.The blasts happened in the Wusasa and Sabongari districts of Zaria at the Christ the King Catholic Cathedral and ECWA GoodNews Church, residents and NEMA said. "A total of 16 persons have so far been confirmed as a result of the explosions within the three churches," police said in a statement.Several residents in Sabongari said the Catholic church was badly damaged by the blast. "I went close to the church but could not access it due to heavy police and military security deployed around it," resident Mahmud Hamza told AFP."From where I stood I could see a badly destroyed church still burning from the explosion. It is obvious there were deaths from the scale of the damage and the fire," he added.Another resident spoke of bodies being taken out. Residents of Wusasa district spoke of many injuries but could not confirm any deaths at the ECWA GoodNews Church."Many people in the church were injured but I have not seen any dead bodies," a woman who was in the church in Wusasa at the time of the explosion said by telephone from her hospital bed.NEMA officials said the third attack hit the Sharon Church in Kaduna city and that there were two other bombings in the Christian-dominated towns in the south of the state.Emergency officials reported attacks in Nassarawa and Barnawa in the south of the same state but police did not confirm them and the targets remained unclear.Southern Kaduna is some four kilometres (three miles) from Kaduna city and about two hours by car from Zaria.The bomb blasts triggered violent protesting by Christian youth mobs, who barricaded roads in the towns of Trijania, Gonin Gora and Sabon Tasha, attacking motorists who looked Muslim."I cancelled my trip to Abuja because of the huge number of rioters that have taken over the roads," one man told AFP.No group has claimed responsibility for the attacks, the latest in Africa's most populous nation and largest oil producer, by the church blasts resembled those previously claimed by Boko Haram.The Islamist group claimed a suicide bombing at a church in Bauchi State that killed at least 15 earlier this month.The group is responsible for more than 1,000 deaths since mid-2009.