(Onlookers gather near the bomb-damaged Shalom Church in the northern Nigerian city of Kaduna June 17, 2012. REUTERS/Stringer)

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”.