The new unrest broke out in the cities of Kaduna and Damaturu, adding to fears of spiralling violence in the country's north, where Islamist group Boko Haram's insurgency has been concentrated.

With anger boiling over among Christians after the latest attacks on churches on Sunday, there have been renewed concerns over whether the violence could lead to a wider sectarian conflict in a country roughly divided between a mainly Muslim north and mostly Christian south.

The violence over the last three days has included three suicide bombings at churches in Kaduna state on Sunday which killed at least 16 people.

Those bombings sparked reprisals by Christian mobs with machetes who roamed the streets of the state capital, also called Kaduna, burning mosques and killing at least 36 people.

On Monday and Tuesday, explosions and shootings rocked the northeastern city of Damaturu, leaving at least 34 people dead, according to a hospital source.

The unrest in Damaturu, hit previously by heavy violence blamed on Boko Haram, included gun battles between suspected Islamists and security forces.

Fresh rioting also broke out in Kaduna on Monday night and Tuesday, including protests after Muslim residents were said to have been turned away from a morgue where they went to collect the remains of relatives.

Some of the rioters carried guns and shots could be heard, while cars and shops were burnt, according to residents.

At least nine people were killed between Monday night and Tuesday in Kaduna, according to the spokesman at St. Gerard's Catholic hospital, Sunday John Ali.

The country's main Christian body issued a statement on Tuesday saying attacks attributed to Boko Haram suggested a "systematic religious cleansing," adding that the Islamists had "declared war on Christians".

The Christian Association of Nigeria also harshly criticised the government over what it described as a befuddled response to Boko Haram's insurgency.

"Since these terrorist acts began, nothing the president, Dr. Goodluck Ebele Jonathan, has done has been reassuring that the end to this spate of bombings and gun attacks is in sight," the statement said.

"On the contrary, his utterances after each bombing and killings, even if unwittingly, seem to have cast a hallmark of weakness on his presidency and an escalation of the terrorist acts."

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With the president traveling to Rio on Tuesday to attend a UN environmental summit, Vice President Namadi Sambo met security chiefs in the capital Abuja.

National Security Adviser Owoye Azazi told journalists afterward that the Kaduna state governor was speaking with religious leaders in a bid to address the violence.

He said the strategy was that "church leaders can talk to the leaders in the mosques, to see how they can tell the adherents to please be mindful that we need to address the security situation in the country peacefully."

Both Kaduna and Damaturu were under round-the-clock curfews as security forces worked to restore calm.

As their insurgency has intensified, Boko Haram's demands have varied, prompting speculation that the group is composed of disparate cells, including a hardcore Islamist wing.

The extremists have previously said they intended to create an Islamic state across the mainly Muslim north of Nigeria, Africa's most populous nation of 160 million and the continent's largest oil producer.

Members of the group have sought training in northern Mali from Al-Qaeda's north African branch.AFP