Boko Haram, which roughly translates as “Western education forbidden,” seeks to impose a stricter application of Shariah , or Islamic, law across the largely Muslim northern half of Nigeria. For more than two years it has been waging an insurgency against the government, mainly through bombings. Hundreds have been killed in these attacks, mostly in the north, many of them police officers, government officials and military personnel.

Critics of the government campaign against Boko Haram say that the effort has not only failed but has increased the sect’s appeal, because the security forces’ heavy-handed tactics have given it new sympathizers.

The sect’s attacks have been further bolstered by festering economic resentment in the impoverished and relatively neglected north, which has an exploding birthrate, low levels of literacy and mass unemployment.

Boko Haram’s campaign has grown in intensity and scope all year. Attacks on the national police headquarters in Abuja in June and a suicide bombing at the United Nations building there in late August, which killed at least 23, signaled an expansion in the group’s focus. Experts viewed those attacks as worrisome indicators that Boko Haram was receiving training outside Nigeria, perhaps from Al Qaeda’s North African affiliate.

The obvious religious overtone to the attacks on Christmas Day appeared to represent another dangerous shift in strategy.

“This is a sign that they’ve decided to try to ignite the Muslim-Christian fault line,” said Darren Kew, a Nigeria expert at the University of Massachusetts Boston .

If so, it has ominous implications, because hundreds have died in clashes between Nigeria’s Muslims and Christians in recent years. That fight has taken place largely outside Boko Haram’s base in the north.

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“They are trying to expand from their roots,” said Mr. Kew, adding that until now, the group had “largely avoided antagonizing the Christians. This is a strategic choice on their part to broaden their offensive.”

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Catholic officials in Nigeria expressed concern Sunday about the Madala bombing. The White House also issued a statement deploring the violence.

“It’s very worrying,” said Archbishop Ignatius A. Kaigama of Jos, where there was a smaller explosion at a church on Sunday. No casualties were reported, but the blast occurred in a violence-wracked city on the religious fault line between north and south. “With this happening, it’s a different dimension,” the archbishop said.

A Boko Haram spokesman, identified as Abu Qaqa, claimed responsibility for the attacks in statements to the local news media.

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The night before, there was an attack at a church in the northern town of Gadaka, just outside Damaturu. Worshipers’ cars were set on fire while the owners were inside the church, The Sunday Trust, a northern Nigerian newspaper, reported. There were no casualties in that attack.

Damaturu was the scene of heavy fighting between Boko Haram and security forces on Thursday and Friday. The army said it had killed more than 50 members of the sect, and that three soldiers died.

Boko Haram claimed responsibility for an attack in Damaturu in November that killed more than 100 people.

Witnesses in Madala described a hellish scene of fire confusion, late and inadequate response, and broken bodies.

“Mass just ended and people were rushing out of the church and suddenly I heard a loud sound — bam!” one witness, Nnana Nwachukwu, told Reuters. “Cars were in flames and bodies littered everywhere.”

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A parishioner, Timothy Onyekwere, told Reuters that he was in the church with his family when the bomb exploded.

“I don’t know how many were killed, but there were many dead,” he said.

Mr. Abati, the government spokesman, said in an interview that President Goodluck Jonathan was determined to “upgrade the performance of the security agencies” in response to the wave of attacks. “Security is taking the No. 1 position in the 2012 budget,” he said.

“The president is in a very sad mood,” he said. “These are nihilists. These are people who just want to pull down the country.”

Yet it is precisely the actions of the security services — the police and military — that have been called into question all year, as they have responded to attacks with ruthless neighborhood sweeps, and what critics describe as indiscriminate repression.

“Operations by the security agencies are eliminating people and driving them into the arms of extremists and terrorists,” said Chidi Odinkalu, senior legal adviser in the Open Society Institute office in Abuja, citing a harsh crackdown in Yobe State, where Damaturu is located.

Western security officials have also been critical of the government’s response to Boko Haram — noting, for example, that Nigerian officials have little useful intelligence. “It’s just a waiting game until the state puts in place a pro-active network or infiltrates the group,” one such security official said in a recent interview. He asked not to be identified because of the delicacy of his position.

“They’ve had more losses than wins,” the official said. “Their methods are primitive — checkpoints,” he said. “It’s sad to me that they can’t seem to do anything more substantive.”