Explosions at three churches in northern Nigeria have killed at least seven people, prompting Christian youths to drag Muslims out of their cars and kill them in retaliation, officials and witnesses said.

There was no immediate claim of responsibility for the blasts on Sunday but the Islamist sect Boko Haram has often attacked church services in Nigeria.

Two explosions within minutes of each other rocked churches in the town of Zaria, in Kaduna state. First, a suicide bomber drove a blue Honda Civic into a church, burning the front entrance and damaging the building, the church's pastor said.

"Three people are confirmed killed. Others have been taken to hospital for treatment," the Rev Nathan Waziri said.

Militants 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 militants were later caught by a mob and killed.

A blast hit a third church in the state's main city of Kaduna, causing an unknown number of casualties, witnesses and the National Emergency Management Agency said.

After the bombs, youths blocked the main road leading south out of Kaduna to the capital, Abuja, dragging Muslims out of their cars and killing them, witnesses said.

"We had to return home when we saw them [the Christian youths] attacking. I saw many bodies on the ground but I don't know how many were dead or just injured," said Rafael Gwaza, a Kaduna resident.

Another witness, Haruna Isah, said up to 20 people may have been killed in reprisals at the roadblock. "There were bodies everywhere on the ground," he said.

Regular attacks on Sunday church services are usually claimed by Boko Haram, which says it is fighting to reinstate an Islamic caliphate that would adhere to strict sharia law.

The group, which has become increasingly radicalised and meshed with other Islamist factions in the region, including al-Qaida's north African wing, is the main security threat to Nigeria.

Islamist militants also attacked two churches in Nigeria last Sunday, 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.

The Islamists' leader, Abubakar Shekau, has justified attacks on Christians as 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.