Nigeria has launched a military campaign to flush Islamist militants out of bases in the north-east of the country.
Large numbers of troops have been deployed in a bid to end an insurgency by the Boko Haram group that has seized control of significant parts of the area.
President Goodluck Jonathan has declared a state of emergency in three regions – Borno, Adamawa and Yobe – following attacks by Boko Haram militants.
A government spokesman said: "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 our borders."
The insurgency has cost thousands of lives and destabilised Africa's top energy producing nation since it began in 2009. Boko Haram has targeted the security forces, Christian worshippers and politicians in Nigeria's mainly Muslim north.
The troop deployment is likely to placate some of Mr Jonathan's critics, who had accused him of not facing up to the gravity of the crisis, although some northern politicians have 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.
Six trucks carrying soldiers were seen entering 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. One resident said: "What I saw scared me. I have never seen soldiers on the move quite like this before."
Another voiced widespread fears that civilians could be caught up in the conflict. He added: "There is a lot of apprehension about the state of emergency."
Mr Jonathan announced the emergency after growing evidence that a better-equipped, better-armed Boko Haram now controls territory around Lake Chad.
The President said: "What we are facing is a rebellion and insurgency by terrorist groups. Already, some northern parts of Borno state have been taken over by groups whose allegiance is to different flags and ideologies."
Officials said 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.
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.
Rights groups say abuses by Nigerian troops in the north-east have alienated the population against them.
On April 16, scores were killed in the 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.
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