Nigerian forces have been attacking Islamist camps in the northeast in the biggest offensive since Boko Haram began its insurgency four years ago.
Fighter jets and attack helicopters have been used. A military source said at least 30 rebels had been killed. Ground operations are due to be carried out.
It follows an upsurge of violence against government and Christian targets by Boko Haram militants who want an Islamic state.
Earlier this week President Goodluck Jonathan declared a state of emergency across three northeastern border regions, where 2,000 people have died in the four-year insurgency.
Some observers back tough action against the rebels.
“These people have gone beyond the pale of reason and for me and for the majority of Nigerians enough is enough, and I think the president has also reached that state of mind,” said Derin Ologbenla, a political scientist at the University of Lagos.
He added that he thought the state of emergency should be extended to more states where Boko Haram was operating.
“The president has been very patient with them to come out and make their claims or let us know their grievances and they have not really come out to let us know their grievances. What they have done is just to show brutal force,” he said.
Last week insurgents killed dozens of police, soldiers and prison staff as they stormed at jail in Borno state and freed 100 inmates.
The army offensive follows growing evidence that Boko Haram now controls parts of the northeast. It is the military’s most serious operation since 2009, when 800 people were killed.
However the US has warned against a “cycle of violence”, a view echoed by human rights groups such as Amnesty International and Human Rights Watch who have documented abuses by Nigerian forces.