Suicide bombers killed 21 people in attacks on three churches in Nigeria during Sunday services, exacerbating religious tensions in a West African nation that is almost evenly divided between Muslims and Christians.

Authorities arrested one of the bombers who survived, said Kaduna State police chief Mohammed Abubakar Jinjiri, but he declined to say who police suspect was responsible for the bombings.

It was the third Sunday in a row that deadly attacks have been carried out against Christian churches in northern Nigeria. No one immediately claimed responsibility for the latest one, but suspicion fell on the radical Islamist sect Boko Haram because it took responsibility for the two earlier weekend assaults.

Boko Haram is waging an increasingly bloody fight with security agencies and the public in Nigeria. More than 560 people have been killed in violence blamed on the sect this year alone, according to an Associated Press count.

On Sunday, the suicide bombers drove explosive-laden cars to the gates of two churches in different parts of the city of Zaria and detonated them within minutes of each other. A similar attack targeted a church in the city of Kaduna about half an hour later at about 9:25 a.m., police said, prompting reprisals by Christian youths.

The attacks in the northern state of Kaduna killed a total of 21 people and wounded at least 100, said an official who works with a relief agency involved in rescue efforts. He spoke on condition of anonymity because he wasn't authorized to speak to journalists.

It wasn't immediately known if the reprisal attacks caused casualties in the religious flashpoint state, but hundreds of people have died in previous retaliatory violence there.

"The Boko Haram group's intention in bombing the churches is to attract reprisal attacks from the Christians, draw the battle line between Muslim and Christians and, (by doing so) get moderate Muslims to support them," said Shehu Sani, the president of the Kaduna-based Civil Rights Congress of Nigeria.

In Italy, the Vatican decried what it called systematic attacks against Christian churches in Nigeria. Vatican spokesman the Rev. Federico Lombardi said in a statement that the "systematicness" of the attacks against Christian places of worship on a Sunday is "horrible and inacceptable" and reflects "an absurd design of hatred."

Within an hour of the Kaduna city attack on Shalom Church, an Associated Press reporter saw billows of smoke over a mosque in a predominantly Christian part of the city. Some Christian youths quickly mounted illegal roadblocks and were seen harassing motorists. A motorcycle taxi rider in that same neighborhood lay seriously hurt and bleeding by the road side. Motorcycle taxi riders there are often presumed to be Muslim and become easy targets during reprisal attacks by Christians.

"The Christians can't see Boko Haram," said Sani, "so they'll retaliate against Muslims."

Churches have been increasingly targeted by violence in Nigeria, with Boko Haram claiming some of the attacks. The situation has led churches in Nigeria's predominantly Muslim north to boost their security in a nation of more than 160 million people.

Last weekend, a suicide car bomber detonated his explosives outside a church in the central Nigerian city of Jos as gunmen attacked another church in the northeastern city of Biu, killing at least six people and wounding dozens. Boko Haram claimed responsibility for the two attacks.

The weekend before that, at least 15 people were killed and dozens more wounded after a suicide car bomber drove into Living Faith Church's compound in the northern Nigeria city of Bauchi and detonated his explosives as worshippers left an early morning service.

Earlier, an Easter Day blast in Kaduna left at least 38 people dead, and Christmas Day suicide bombing of a Catholic church in Madalla near Nigeria's capital killed at least 44.

For now, Christian leaders in the north are encouraging Christians to keep attending Sunday services.

"They can only destroy the flesh," said Sunday Aibe, a spokesman for the Northern Christian Association, "and not the spirit."

But many feel that going to church puts them at risk.

Timothy Musa, an unemployed 25 year old, says he won't go to church unless he sees the security improve.

"Unemployment in the country is still on the rise and now there is the insecurity of lives and property. How can I?" said the member of Christ the King Catholic Church, one of the three targeted by Sunday's attacks.

These latest attacks have occurred in the place where they are the most likely to have a ripple effect.

"Targeting religious places of worship is always an inflaming situation in a divided society such as we have here," said criminologist Innocent Chukwuma.

Kaduna state, which sits on Nigeria's dividing line between its largely Christian south and Muslim north, has a history of religious tensions.

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