Nigeria launched a military campaign on Wednesday to flush Islamist militants out of bases in its border areas after President Goodluck Jonathan declared a state of emergency in the northeast.
Nigerian troops moved into the region in large numbers, part of a plan to rout an insurgency by the Boko Haram Islamist group that has seized control of significant parts of the region.
"The operations, which will involve massive deployment of men and resources, are aimed at asserting the nation's territorial integrity and enhancing the security of ... all territories within Nigeria's borders," a statement from Defense Headquarters said.
Residents and Reuters reporters saw army trucks carrying soldiers enter Yola and Maiduguri after Jonathan declared the emergency on Tuesday in three states - Borno, Adamawa and Yobe - following attacks by Boko Haram militants.
The insurgency has cost thousands of lives and destabilized Africa's top energy producing nation since it began in 2009. Boko Haram have targeted the security forces, Christian worshippers and politicians in Nigeria's mainly Muslim north.
The troop deployment is likely to placate some of Jonathan's critics, who had accused him of not facing up to the gravity of the crisis, although some northern politicians have already voiced concerns over the ratcheting up of tensions.
It is also unclear whether greater military might can win a battle against an adversary that has proved a master at melting away under pressure, only to re-emerge again elsewhere.
Military officials in the northeast and at headquarters in the capital Abuja were not immediately available for comment.
A Reuters reporter saw six trucks carrying soldiers enter Yola, the capital of Adamawa state. In the Borno state capital Maiduguri, the biggest city in the area and birthplace of the insurgency, residents also reported an influx of troops.
The mood was tense in that city. Shops were mostly shut and there were few people on the streets. Schools were closed.
"What I saw this morning scared me," said one man in Maiduguri, Ahmed Mari. "I have never seen soldiers on the move quite like this before."
Another, Kabir Laoye, voiced widespread fears that civilians could be caught up in the conflict: "There is a lot of apprehension about the state of emergency," he said.
Jonathan announced the move in a televised address on Tuesday.
His orders followed growing evidence that a better equipped, better armed Boko Haram now controls territory around Lake Chad, where local officials have fled.
"What we are facing is ... a rebellion and insurgency by terrorist groups which pose a very serious threat to ... territorial integrity," Jonathan said in the address. "Already, some northern parts of Borno state have been taken over by groups whose allegiance is to different flags and ideologies."
Officials say militants control at least 10 local government districts of Borno state -- a semi-desert region that once hosted one of West Africa's oldest medieval Islamic empires -- and are using porous borders with Cameroon, Chad and Niger to smuggle in arms and mount increasingly bold attacks.
Security sources say their strategy appears to be similar to that of the al-Qaeda-linked militants who overran Mali late last year, before the French kicked them out in January: take over remote desert areas and establish a de facto rule there, then use that as a base from which to expand.
Growing links with jihadists across the Sahara region, and the fallout from Libya's war, are giving Boko Haram better access to weapons, funding and training.
Dozens of Boko Haram fighters laid siege to the Borno town of Bama last week, freeing more than 100 men from prison and leaving 55 people dead, mostly police.
Some officials in from Borno's government doubted the state of emergency would work unless security forces can win popular support.
"This state of emergency will not change anything if the people do not cooperate and start exposing members of Boko Haram," said David John, a director in the state government.
Rights groups say abuses by Nigerian troops in the northeast have alienated the population against them.
A crackdown on Boko Haram in 2009 led to the deaths of 800 people, including its founder Mohammed Yusuf, who died in police custody. Instead of crushing them, it unleashed a torrent of popular rage that only made the Islamists more deadly.
On April 16 this year, scores were killed in the fishing village of Baga, on Lake Chad, when troops from Nigeria, Niger and Chad raided it looking for Islamists who had killed a soldier. Residents said soldiers were responsible for many civilian deaths, triggering widespread anger towards the army.