MAIDUGURI/YOLA, Nigeria Nigeria launched a military campaign on Wednesday to flush Islamist militants out of their bases in remote border areas, after President Goodluck Jonathan declared a state of emergency in the northeast.
Nigerian troops deployed in large numbers, part of a plan to rout an insurgency by the Boko Haram Islamist group that has seized control of parts of the region.
"The operations, which will involve massive deployment of men and resources, are aimed at asserting the nation's territorial integrity," a Defence Headquarters statement said.
The campaign targets semi-desert areas of three states in which Jonathan declared an emergency on Tuesday - Borno, Adamawa and Yobe, three of the country's poorest and most remote.
The Islamist insurgency has cost thousands of lives and destabilised Africa's top energy producer since it began in 2009, but it has mostly happened far from economic centres such as Lagos. The capital Abuja was, however, bombed in 2011 and 2012.
It has not affected southern oil fields that provide the bulk of government revenues in Africa's second biggest economy.
Residents and Reuters reporters saw army trucks carrying soldiers enter Yola and Maiduguri to seek out militants from Boko Haram, whose rebellion has targeted the security forces, Christians and politicians in the mainly Muslim north.
The deployment will placate some of Jonathan's critics, who had accused him of not facing up to the gravity of the crisis, but northern politicians voiced concerns over rising tensions.
The United States said it feared a worsening cycle of violence that threatens Nigeria and the West Africa region.
It is for now unlikely tensions will boil over to other parts of the country. The Islamists have a foothold across most of the north, but nothing like the power base they have established in these three states.
In December 2011, Jonathan declared a state of emergency over some local government areas after a church bombing blamed on Boko Haram killed 37 people, but he lifted it in July 2012.
Ayo Oritsejafor, head of the powerful Christian Association of Nigeria, said the move showed Jonathan's plan to offer the rebels an amnesty had been misguided, saying "no reasonable agreement can be reached with terrorists".
Yet it is 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 elsewhere.
"The government is thinking it can crush them like Sri Lanka crushed the Tamil rebels," Kole Shettima, chairman of the Centre for Democracy and Development, told Reuters.
"But in Sri Lanka they pushed them to the water, whereas here they will just flee into the desert and come back."
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 U.S. State Department, which has criticised the Nigerian military's heavy handed tactics, said it was "deeply concerned about increasing insecurity in northern Nigeria and the potential threat it poses to both Nigeria and the region."
"The declaration of states of emergency ... reflects the worsening cycle of violence in northern Nigeria," State Department spokesman Patrick Ventrell said.
Maiduguri residents are used to living under military restrictions - a curfew kills activity at 6 p.m. every day - and soldiers patrolling the streets are a common sight. Most shops and schools were shut.
"I have never seen soldiers on the move quite like this before," said one man in Maiduguri, Ahmed Mari.
Jonathan's 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."
Officials say militants control at least 10 local government districts of Borno state, which once hosted one of West Africa's oldest medieval Islamic empires, and are using porous borders with Cameroon, Chad and Niger to smuggle in bigger arms.
Dozens of Boko Haram fighters struck the town of Bama last week, leaving 55 people dead, mostly police.
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.
They have forged growing links with jihadists across the Sahara region, intelligence sources say. But they also enjoy a degree of support among a poor, ill-educated populace.
"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, such as a raid in the remote town of Baga that killed dozens last month, have alienated the population.
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.
(Additional reporting by Lanre Ola in Maiduguri; Writing and additional reporting by Tim Cocks in Lagos; Editing by Alastair Macdonald and Will Waterman)