MAIDUGURI, Nigeria (Reuters) - Nigerian forces used jets and attack helicopters to bombard militant camps in the northeast on Friday, their biggest offensive since Boko Haram began an insurgency almost four years ago to try create a breakaway Islamic state.
A military source said at least 30 insurgents had been killed in the operation, which the defence headquarters spokesman said included the Sambisa game reserve in Borno state, centre of the uprising and heart of a medieval Islamic empire.
"It is not just Sambisa, every camp is under attack," the spokesman, Brigadier-General Chris Olukolade, said by telephone. "But we have not done the mopping up operations on the ground to determine the numbers killed."
Nigerian forces are trying to regain territory controlled by well-armed Boko Haram Islamist insurgents 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.
More troops arrived in the Borno state capital Maiduguri on Friday, witnesses said.
"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, life was slowly returning, with traffic back on the roads and shops re-opening, as many of the military operations take 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 housing 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.
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," 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."