A radical Islamist sect claimed responsibility Thursday for attacks that left at least 40 people dead in a northeast Nigerian city this week, as the government struggles to corral rising sectarian violence.

An email statement attributed to Boko Haram and obtained by The Associated Press on Thursday said it launched the attacks in the city of Damaturu.

The attacks began Monday evening with blasts targeting police and military targets in Damaturu, authorities said. Gunfire echoed across the city for hours, spilling into Tuesday afternoon. One resident said at least two schools were torched.

The attacks killed at least 34 civilians and six security officers including four policemen and two soldiers, according to Yobe State police chief Patrick Egbuniwe. The casualty figure could be higher since Muslim mourners often quickly bury their dead without alerting authorities.

"The terrorists are trying to show that they can't be stopped," Egbuniwe said soon after the attacks started.

The Islamist Boko Haram sect, whose name means "Western education is sacrilege" in the Hausa language, is waging an increasingly bloody fight with Nigeria's security agencies and public. More than 640 people have been killed in violence blamed on the sect this year alone, according to an AP count.

"We assure all that the success of the Damaturu operation is really a sign that very soon, Allah will give us the chance of overthrowing this unjust and heathen government and replacing it with an Islamic system which is just," the statement attributed to the group said in the local Hausa language.

Boko Haram's spiritual home of Maiduguri is about 80 miles (130 kilometers) east of Damaturu, the capital of rural Yobe state.

The Yobe state government relaxed a 24-hour curfew in Damaturu on Thursday after the city returned to relative calm. Damaturu had already been under a partial curfew since a state of emergency was declared in December. Last November, the Islamist sect claimed responsibility for bombings and shootings that left more than 100 dead in and around Damaturu.

Nigeria, a nation of more than 160 million people, almost evenly divided between Muslims and Christians. The north is predominately Muslim while the south is Christian.

Boko Haram on Sunday carried out a trio of church bombings in Kaduna state in north-central Nigeria, sparking reprisal killings that are ongoing.

A relief agency official involved in rescue efforts said Wednesday that at least 98 people have died in the church blasts and ensuing reprisals. He spoke anonymously as he said he was not authorized to speak to journalists.