Nigerian troops and police are hunting for the remnants of Boko Haram, an Islamist group that went on a killing spree in the country's north.
At least 30 people were killed in fresh clashes between security forces and the group the northern state of Yobe on Wednesday, a police source said.
"Thirty have so far been killed in Hawan Malka," the AFP news agency quoted the source as saying, referring to an area outside Potiskum, Yobe's second largest city.
Wednesday's violence came after the army shelled a mosque and the home of Mohammed Yusuf, the group's alleged leader, in Maiduguri, the capital of Borno state.
"We are not sure whether he has been killed in the shelling or has managed to escape," a police officer said of Yusuf.
Boko Haram opposes western-style education and has said it wants to lead an armed insurrection and rid society of "immorality" and "infidelity".
About 140 people have been killed in three days of violence in Nigeria's Muslim-dominated north.
Umaru Yar'Adua, Nigeria's president, has vowed that the group will be hunted down and punished.
He said that the military operation currently under way would "contain them once and for all".
"They will be dealt with squarely and forthwith," he said.
Before leaving on a trip to Brazil on Tuesday, Yar'Adua said that the situation was "under control".
But fresh fighting broke out in Maiduguri following the assault on the home of Yusuf.
Dozens of people took shelter from the bombardment in a local police station.
"It is the first time in my life that I hear this kind of mortar shelling," said one man, who had taken cover there, along with his wife and three daughters.
"I thought they targeted my house."
An AFP correspondent reported witnessing soldiers shooting three young men dead at point blank range close to the city's police headquarters.
The men, who had just been arrested, were seen kneeling and pleading for their lives before being shot.
"There has been a serious intensification of the assault on members of this group, Boko Haram, which is behind this wave of killings," Yvonne Ndege, Al Jazeera's correspondent reporting from Abuja, Nigeria's capital, said.
"The president of Nigeria has said that anybody perpetrating violence will be dealt with very, very severely - in fact, that means imminent death," she said.
"If you're caught working among Boko Haram fighters, there is absolutely no question, your life will not be spared."
Boko Haram, which means "Western education is prohibited" in the local Hausa dialect, has called for the enforcement of sharia or Islamic law, across Africa's most populous nation.
But Nii Akuetteh, the founder of the Democracy and Conflict Research Institute, an African think-tank, told Al Jazeera that, while religious clashes had occurred in the past in Nigeria, the recent clashes appeared to have little political motivation.
"Previously when you had religion rear its head in politics [in Nigeria] you had a clash between Christians mainly in the south and Muslims in the north.
"I think that one you have to talk of the political implications of that, but the most recent, frankly, it seems to me is nothing but religious extremism and violence."
Nigeria's 140 million people are nearly evenly divided between Christians, who dominate the south, and the primarily northern-based Muslims.
Islamic law was implemented in 12 northern states after Nigeria returned to civilian rule in 1999 following years of military rule.
Akuetteh also said that poverty, which has sparked conflict elsewhere in Nigeria, mainly in the oil-rich Niger delta, was not a contributing factor.
"I think religious politicisation of religion in Nigeria is separate and apart from the poverty that is there.
"I would look more to religious prejudice and extremists wanting to inject religion into politics rather than poverty per se."
The clashes began on Sunday in nearby Bauchi state, with fighters attacking police stations, before spilling over into Yobe. Officials said that 55 people were killed in both states.
Residents said fighters armed with machetes, knives, bows and arrows and home-made explosives, attacked police buildings and anyone resembling a police officer or government official in the city.
But most of the casualties appear to have been in Maiduguri, the northeastern city known as the birthplace and stronghold of the group.