ZARIA, Nigeria (Reuters) - Bombings at three churches in Nigeria's northern Kaduna state killed at least seven people and wounded others on Sunday, 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 the Islamist Boko Haram group has often attacked church services in Nigeria, Africa's most populous country and split roughly evenly between Christians and Muslims.
The bombings and retaliatory attacks stoked fears of wider sectarian conflict in Nigeria, Africa's top oil producer and an OPEC member, although flare-ups of this nature have usually been brief in the past.
Kaduna state's governor declared a 24-hour curfew in a bid to restore calm. Boko Haram, increasingly radicalised and meshed with other Islamist groups in the region, including al Qaeda's north African wing, is the leading security threat to Nigeria.
Two blasts rocked churches in the town of Zaria within minutes of each other. First, a suicide bomber drove a blue Honda civic car into a 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.
Then, attackers threw bombs at another church, killing four children who were playing on the streets outside, said Deborah Osagie, who lives opposite the church. She said the attackers were later caught by a crowd and killed.
A blast hit a third church, called the Shalom Church, in the state's main city of Kaduna, witnesses said.
"There are a number of injured and dead from the bombings. We don't have precise figures yet," said Musa Ilela, an official from the National Emergency Management Agency in Kaduna state.
"The injured and dead have been moved to the hospitals. Our men have not been able to get to the blast site in Kaduna."
After the bombings, Christian youths blocked the highway leading south out of Kaduna to the Nigerian 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."
Last Sunday, Islamist 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, 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 Ralph Gowling)