By Isaac Abrak
ZARIA, Nigeria (Reuters) - Suicide car bombers attacked three churches in northern Nigeria on Sunday, killing at least 19 people and wounding dozens, and triggering retaliatory attacks by Christian youths who dragged Muslims from cars and killed them, officials and witnesses said.
There was no immediate claim of responsibility for the bombings but Islamist group Boko Haram has often attacked church services in Nigeria, Africa's most populous country, split roughly evenly between Christians and Muslims.
The violence stoked fears of wider sectarian conflict in Nigeria, an OPEC member and Africa's top oil producer where the Christian Association of Kano, northern Nigeria's main city, called the bombings "a clear invitation to religious war".
Last Sunday, militants attacked two churches in Nigeria, spraying the congregation of one with bullets, killing at least one person, and blowing up a car in a suicide bombing at the other, wounding 41. Boko Haram claimed responsibility.
In the latest violence, the first two blasts rocked churches in the town of Zaria within minutes of each other.
First, a suicide bomber drove a blue Honda Civic into Ekwa Church, its pastor told a Reuters cameraman at the scene.
"Three people are confirmed killed. Others have been taken to hospital for treatment," said Reverend Nathan Waziri.
The second suicide car bombing was at Kings Catholic Church, killing 10 people, said Bishop of Zaria George Dogo who was giving a service in the church when it was attacked.
Suicide bombers in a Toyota saloon then hit Shalom Church in the state's main city of Kaduna, killing six people. The military said the dead included an army sergeant.
Manan Janet, who was in the church, said she saw six bodies. "It was terrible. I'm traumatized," she said.
Musa Ilela, an official from the National Emergency Management Agency in Kaduna state, said the death toll had yet to be established.
"The injured and dead have been moved to the hospitals. Our men have not been able to get to the blast site in Kaduna," he said.
After the bombings, Christian youths blocked the highway leading south out of Kaduna to the capital Abuja, pulling Muslims out of cars and killing them, witnesses said.
"We had to return home when we saw (the Christian youths) attacking. I saw many bodies on the ground, but I don't know how many were dead or just injured," said Kaduna resident Rafael Gwaza.
Witness Haruna Isah said up to 20 people might have been killed in reprisals at the road-block. "There were bodies everywhere on the ground," he said.
Kaduna state governor Patrick Yakowa called for calm.
"In view of the incidents and the need to have complete normalcy and to forestall a further break down of law and order, the state government has imposed a 24-hour curfew in the whole state," a statement from his office said.
"The state government considers this to be necessary in order to avert further loss of lives and properties."
Boko Haram says it is fighting to reinstate an ancient Islamic caliphate that would adhere to strict sharia, or Islamic law.
The Islamists' leader Abubakar Shekau says attacks on Christians are in revenge for killings of Muslims in Nigeria's volatile "Middle Belt", where the largely Christian south and mostly Muslim north meet.
Kaduna is close to the Middle Belt areas. (Additional reporting by Ibrahim Mshelizza and Garba Mohammed in Kaduna, Mike Oboh in Abuja and Tim Cocks in Lagos; Writing by Tim Cocks; Editing by Robin Pomeroy)
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