Story highlights Pope Benedict XVI addresses violence in the region

One attack was in Yobe province, the other in Borno state

A militant Islamist group has been targeting Christians and some Muslims

There was a wave of attacks last year on Christmas

At least 12 people died in northern Nigeria when attackers raided two churches during Christmas Eve services, police said.

One assault occurred at the Church of Christ in Nations in Postikum, in Yobe province. Gunmen attacked worshipers during prayer, killing six people, including the pastor, and setting the building on fire

Worshipers also were attacked at the First Baptist Church in Maiduguri, in Borno state. A deacon and five church members were killed.

They were the latest strikes against Christians in the region. More than 30 people died in a wave of Christmas Day attacks in the north last year, blamed on Boko Haram, a militant group that has targeted Christians and Muslims it considered insufficiently Islamist.

Pope Benedict XVI referred to the northern Nigerian violence in his traditional Christmas message from Vatican City on Tuesday.

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"Savage acts of terrorism" in the region, he said, "continue to reap victims, particularly among Christians."

In October, a report from Human Rights Watch also addressed violence in northern Nigeria, particularly from Boko Haram.

"Suspected members of the group have bombed or opened fire on worshipers in at least 18 churches across eight northern and central states since 2010. In Maiduguri, the group also forced Christian men to convert to Islam on penalty of death," it said.

It is not immediately known whether the group was behind the latest attacks.

The Christmas attacks came as families whose kin died in last year's killings delivered graveside prayers for a peaceful holiday period. Nigerian President Goodluck Jonathan issued a statement promising better days next year, including better security.

"Sometimes, challenges make people doubt the sincerity of government, but I am confident that God knows everything," he said Sunday.

But residents told CNN that despite assurances of security, they have been attacked again.