An Islamic group has claimed responsibility for Christmas Day church bombings in Nigeria, which killed 39 people.

Explosions ripped through five churches across the African country as Christians celebrated the birth of Jesus Christ.

The spokesman for a group calling themselves Boko Haram claimed responsibility for the attacks in an interview with The Daily Trust, the newspaper known across Nigeria's Muslim-dominated north.

"There will never be peace until our demands are met," the newspaper quoted the spokesman as saying.

"We want all our brothers who have been incarcerated to be released; we want full implementation of the Sharia system, and we want democracy and the constitution to be suspended," he said.

A recent U.S. congressional report flagged the group as an "emerging threat" because of of its "rapid rise" and possible links to al Qaeda.

U.S. officials said the violence underscores the increasing ability of terror groups to carry out complex strikes against multiples targets.

Last year, Islamic groups carried out a series of bomb attacks on Christmas Eve that killed 32 people.

The Christian Association of Nigeria is demanding the government protect its churches and parishioners.