Maiduguri - Warplanes striking militant camps in the northeast of Nigeria has drawn a sharp warning from the United States to respect human rights and not harm civilians.
Troops used jets and helicopters to bombard targets in their biggest offensive since the Boko Haram group launched a revolt almost four years ago to establish a breakaway Islamic state and one military source said at least 30 militants had been killed.
But three days after President Goodluck Jonathan declared a state of emergency in the northeast, U.S. Secretary of State John Kerry issued a strongly worded statement saying: "We are ... deeply concerned by credible allegations that Nigerian security forces are committing gross human rights violations, which, in turn, only escalate the violence and fuel extremism."
The United States is the biggest foreign investor in Africa's most populous nation, notably in its energy sector, and buys a third of Nigeria's oil. Washington "condemns Boko Haram's campaign of terror in the strongest terms", Kerry said, but urged Nigeria's armed forces to show restraint and discipline.
Nigerian defence spokesman Brigadier-General Chris Olukolade said in a statement that troops destroyed several Boko Haram camps and weapons stockpiles in forests around Borno state, epicentre of the uprising and relic of a medieval Islamic empire: "Heavy weapons including anti-aircraft and anti-tank guns were also destroyed in the process," he said.
"The special operations ... resulted in the destruction of much of the insurgents' weapons and logistics such as vehicles, containers, fuel dumps and power generators."
He said the death toll amongst the insurgents would be verified during mopping up exercises in the camps, including in the Sambisa game reserve in Borno state. A military source said at least 30 insurgents had been killed in one operation.
Nigerian forces are trying to regain territory controlled by well-armed militants in remote northeastern stronghold states of Borno, Yobe and Adamawa, put under a state of emergency by President Goodluck Jonathan on Tuesday.
The Islamists, seen as the main security threat to Africa's top oil producer, have been staging bolder attacks since last month, including one on the town of Bama that left 55 dead.
Nigerian authorities fear they are creating an enclave in remote border areas, as al Qaeda linked militants did in the deserts of Mali before the French forced them out in January.
But previous efforts to crush Boko Haram have always proved temporary, forcing them to dissipate into hiding places or across borders, where they wait, regroup and then come back.
The military is already overstretched in the north, by operations against oil theft in the south and foreign missions.
More troops arrived on Friday in the Borno state capital Maiduguri, where Boko Haram was founded as a clerical movement opposed to Western culture, but which after a military crackdown on it killed 800 people, morphed into a full armed rebellion and forged ties with al-Qaeda linked groups in the Sahara.
"I saw more than 20 trucks loaded with soldiers fully kitted for battle towards Marte. I wish them luck in ending this BH (Boko Haram) madness," resident Ahmed Ibrahim said.
A day earlier, 11 trucks of police trained in counter-insurgency had arrived in Maiduguri, security officials said. Mobile phone connections to Borno and Yobe states were cut.
In some parts of Maiduguri, and in Yola, the capital of Adamawa state, traffic returned to roads and shops re-opened, as most military operations took place in remote rural areas. Roads out of the city to such areas were sealed off by soldiers.
Thousands of troops are involved in the offensive - the precise number is a secret - an answer to critics who accuse Jonathan, a southern Christian, of underestimating the severity of the crisis in the largely Muslim north.
Several thousand people have been killed since Boko Haram rose up in 2009 to try create an Islamic state in a nation of 170 million split equally between Christians, the majority in the south, and Muslims, mostly in the north.
The violence has mostly happened far from the commercial hub Lagos or political capital Abuja, and hundreds of miles away from oilfields in the southeast, which has dulled a sense of urgency about it amongst Nigeria's elites.
Beyond the region covered by the state of emergency, gunmen stormed a police station and a bank in Katsina state, the army said, a sign the offensive could provoke violence by smaller militant cells across the north.
It was not clear who carried out the attack.
This is not the first time planes have been used to quell an Islamist uprising. In the 1980s, military leaders used air power to put down religiously inspired protests in the north's main city of Kano, a crackdown that left some 5,000 people dead, according to state media at the time.
The emergency affects semi-desert states of some 150 000 sq km (60,000 sq miles) along Lake Chad - near borders with Niger, Chad and Cameroon - an area home to around 10 million people.
Rights groups are concerned the state of emergency will lead to more of the abuses they have documented by Nigerian forces, and some commentators are concerned that this pushes a political solution to the conflict even further out of sight.
Kerry said: "We urge Nigeria's security forces to apply disciplined use of force in all operations, protect civilians in any security response, and respect human rights and the rule of law."
Last month Jonathan floated the idea of an amnesty, but Boko Haram leader Abubakar Shekau rejected it.
"A state of emergency appears to be a further step toward responding to the crisis in the north through military rather than political means," a former U.S. ambassador to Nigeria, John Campbell, wrote in a blog.
"The brutality of the Nigerian security services appears to generate support for the Islamists."