ZARIA, Nigeria (Reuters) - Suicide car bombers attacked three churches in northern Nigeria on Sunday, killing at least 19 people, wounding dozens and triggering retaliatory attacks by Christian youths who dragged Muslims from cars and killed them, witnesses said.
There was no immediate claim of responsibility for the bombings but just one week ago Islamist group Boko Haram claimed responsibility for deadly church attacks.
The Vatican condemned the "systematic attacks against Christian places of worship" which it said proved the existence of an "absurd plan of hate" 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. The Christian Association of Kano, northern Nigeria's main city, called the bombings "a clear invitation to religious war".
Nigeria's Muslims and Christians mostly co-exist peacefully but periodic flare-ups of sectarian violence have killed hundreds since independence from Britain in 1960. In the past, outbursts of retaliatory violence have been short lived
Last Sunday, Boko Haram 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 says it is fighting to reinstate an ancient Islamic caliphate that would adhere to strict sharia 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.
In the latest violence, in Kaduna state, close to the Middle Belt, two blasts rocked churches in the town of Zaria within minutes of each other.
First, a suicide bomber drove a blue Honda Civic into Ecwa 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.
Manan Janet, who was in the church, said she saw six bodies. "It was terrible. I'm traumatised," she said.
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.
"The systematic attacks against Christian places of worship on Sundays is horrible and unacceptable, proof of an absurd plan of hate," the Vatican's chief spokesman, Federico Lombardi, said in a statement.
"The risk that violence will continue is increasing and widening, causing retaliations and heightening tensions," he said, calling for: "effective action to stem and eliminate terrorism for the good of this great country."
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 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 imposed a 24-hour curfew, which largely quelled the unrest.
Police spokesman Frank Mba later issued a statement saying that normalcy had been restored after "a momentary breakdown of law and order" that he blamed on "criminal elements". He put the death toll from the three bombs at 16.
Additional reporting by Ibrahim Mshelizza and Garba Mohammed in Kaduna, Mike Oboh in Abuja, Augustine Madu in Kano, Tim Cocks in Lagos and Steve Scherer in Rome; Writing by Tim Cocks; Editing by Robin Pomeroy