Jos, Nigeria (CNN) -- A string of bombs struck churches in five Nigerian cities Sunday, leaving dozens dead and wounded on the holiday, authorities and witnesses said.
The blasts mark the second holiday season that bombs have hit Christian houses of worship in the west African nation. In a statement issued late Sunday, Nigerian President Goodluck Jonathan called the bombings "a dastardly act that must attract the rebuke of all peace-loving Nigerians."
"These acts of violence against innocent citizens are an unwarranted affront on our collective safety and freedom," Jonathan said. "Nigerians must stand as one to condemn them."
Bombs targeted churches across the country, hitting the cities of Madalla, Jos, Kano, and Damaturu and Gadaka, said journalist Hassan John, who witnessed the carnage in Jos. The death toll in Madalla alone was 18, including two people reported dead overnight at a nearby hospital, Nigeria's National Emergency Management Agency spokesman Yushau Shuaib told CNN.
John said witnesses in Madalla reported a higher death toll, with more than 30 killed. Some victims died after being taken to a hospital, he said.
In Damaturu, a northern town in Yobe state, a police station and a state security building were also bombed, an aid worker said. The worker asked not to be named for security reasons.
Nwakpa Okorie, a spokesman for the Nigerian Red Cross, said the some of the wounded were taken to the capital Abuja for treatment.
"The situation is under control now. The security agents have secured the streets close to the bombed areas ... in Madalla, Jos and Dematuru," he said.
Jonathan said his government "will not relent in its determination to bring to justice all the perpetrators of today's acts of violence and all others before now." And in Washington, the White House said U.S. officials would help Nigeria pursue those behind "what initially appear to be terrorist acts."
"We condemn this senseless violence and tragic loss of life on Christmas Day," White House spokesman Jay Carney said in a written statement. "We offer our sincere condolences to the Nigerian people and especially those who lost family and loved ones."
The first explosion Sunday struck near a Roman Catholic church in Madalla, west of Abuja, Nigeria's capital, the National Emergency Management Agency said. Church officials were trying to get a picture of what happened in the city.
"Lives have been lost but we do not have the details," said the Rev. Michael Ekpenyong. "The area has been cordoned off. I tried to call the priest, but I couldn't get through."
Ekpenyong, the secretary general of the country's Catholic Secretariat, said the church that was bombed was "not a big church, but lots of people attend." Photos from the scene showed burned-out cars and at least three bodies on the ground, one covered with a blanket, at the rural church.
Usman Abdallah Baba, who witnessed the bombing, said there were at least 15 or 16 casualties and that authorities were still counting the toll.
Baba said local people were already blaming the violent extremist Muslim Boko Haram sect, which has targeted Christians as well as Muslims its members consider insufficiently Islamic.
U.N. Secretary-General Ban Ki-moon condemned the acts "in the strongest terms," his office said in a statement Sunday.
He expressed his condolences to the Nigerian people and reiterated a call "for an end to all acts of sectarian violence in the country."
In 2010, five churches in Jos were attacked while residents were celebrating Christmas Eve. The blasts killed dozens in Jos, which lies on a faith-based fault line between the Muslim-dominated north and the mainly Christian south.
On Sunday, two blasts targeted the Mountain of Fire Ministries church in Jos, northeast of the capital, said John. No one was killed in that bombing, which John called a "miracle" -- but a police officer who got into a gun battle with the attackers died of his wounds later, John said, citing officials.
The second church, in Jos, was hit by two explosions when young men threw bombs, John said. Police responded quickly and exchanged gunfire with the attackers, who wounded at least one of the police officers, he said.
The injured officer was rushed to the Jos University teaching hospital for medical attention, but died of his wounds, John said. The attackers fled into the crowd and disappeared after the attack, John said.
Police arrested four people and recovered four unexploded devices, Nigerian state television reported.
Nigeria is Africa's most populous nation and has the world's sixth-largest Christian population -- about 80.5 million people as of 2010, according to a report published this month by the Pew Forum on Religion & Public Life in Washington. That makes the country just over 50% Christian, according to the Pew figures.
The latest attacks follow two days of clashes between militants and security forces in northern Nigeria. Lt. Gen. Azubuike Ihejirika, the Nigerian army chief of staff, said the clashes left three soldiers dead and several more wounded.
The fighting began Thursday between Boko Haram militants and the military in the Yobe state town of Damaturu, Ihejirika said.
"There was a major encounter with the Boko Haram in Damaturu," Ihejirika said Friday. "We lost three of our soldiers, seven were wounded. But we killed over 50 of their members."
Boko Haram translates from the local Hausa as "Western education is outlawed." The group has morphed into an insurgency responsible for dozens of attacks in Nigeria in the last two years.
Boko Haram's targets include police outposts and churches as well as places associated with "Western influence."
CNN's Josh Levs, Richard Allen Greene, Esprit Smith, Karen Smith, Amir Ahmed and Hamdi Alkhshali contributed to this report.