Nigeria's military on Friday attacked Boko Haram Islamist strongholds across the northeast, launching air strikes on insurgent camp with dozens of militants killed in the fighting, the military said.

Several thousand soldiers have spread across three northeastern states where President Goodluck Jonathan imposed a state of emergency, saying Boko Haram had seized territory and declared war on the government.

Forces have also been deployed to seal some of the region's porous borders and block the insurgents from fleeing.

"There have been air strikes since Wednesday," defence spokesman Brigadier General Chris Olukolade told AFP, specifying that they were continuing Friday.

Troops "have destroyed some terrorist camp sites.... Heavy weapons including anti-aircraft and anti-tank guns were also destroyed," he said in a separate statement.

"Dozens of the insurgents have likely been killed," he told AFP, without offering a precise figure.

The offensive is under way in all three states put under emergency decree, including Adamawa and Yobe, but the extremists' traditional base of Borno state is expected to see the most intense fighting.

Many have warned that there is a risk of high civilian deaths and Nigeria's military has been accused of massive rights violations in the past, including indiscriminate attacks on civilians.

The operation is the largest against Boko Haram since 2009, when soldiers flooded Borno's capital Maiduguri, killing more than 800 people and forcing the insurgents underground for a year.

In the town of Gamburu Ngala on the border with Cameroon in northern Borno, residents said that heavily armed troops and tanks arrived on Wednesday, sealing off previously unmanned border posts.

Northeast Nigeria, the epicentre of the Boko Haram insurgency, has porous borders with Cameroon, Chad and Niger, with criminal groups and militants flowing freely between the countries.

"Since January the border posts have been abandoned... but now these posts have been taken over by soldiers," said resident Haruna Garba.

Olukolade confirmed that forces had been sent to the region, but would not say whether the borders had been sealed.

Reports of Boko Haram's presence in Cameroon first emerged in February, following the kidnap there of a French family visiting a game park near the Nigerian border.

The abduction was claimed by Boko Haram and the family was released in April.

-- 'I was so scared for my life' --

Soldiers have surrounded the town of Krenuwa in Marte district, also in northern Borno, one of the areas where Boko Haram took power, chased away all government officials and removed Nigerian flags, residents said.

Abur Kullima told AFP Friday that he fled his home in Krenuwa in fear of the coming assault.

He said that after the state of emergency was declared in a national broadcast late Tuesday, Islamist gunmen began moving through the district trying to mobilise people "in preparation to face Nigerian troops."

"I was so scared for my life and my family's, which led me to decide to leave," he told AFP from Gamburu Ngala, where he is staying with a friend.

Anyone who tries to leave Krenuwa is screened by the soldiers who have encircled the town, he said.

Boko Haram has become notorious for blending in with the local population, both in towns and major cities across the wider north, where they have carried out suicide bombings as well as gun and bomb assaults

Fresh fighting in northern Katsina state, an area not affected by the emergency decree, broke out early Friday, where gunmen attacked police stations and a bank, a police statement said.

Five of the attackers were killed "after a long gun battle," the statement said, not implicating Boko Haram.

The group has largely spared Katinsa, but has carried out scores of attacks in neighbouring states.

Boko Haram says it is fighting to create an Islamic state in Nigeria's mainly Muslim north, but its demands have repeatedly shifted.

Some believe it has sought closer ties to foreign extremist groups like Al-Qaeda's North Africa affiliate, but most analysts believe its agenda is primarily domestic.

Nigeria is Africa's top oil producer but the population remains extremely poor and many of Boko Haram's fighters are believed to be youths radicalised out of frustration with government corruption.

In fighting the Islamists, the United Nations warned Nigeria against using excessive force, which could further "feed local resentment."

The conflict is estimated to have cost 3,600 lives since 2009, including killings by the security forces.