A key city in northeast Nigeria was on lockdown Sunday as the military enforced a 24-hour curfew and blocked supply routes in its sweeping campaign against Islamist insurgents.

The operation against Boko Haram, the group that wants an Islamic state in northern Nigeria, is aimed at retaking territory seized by the militants and ridding the country of "terrorist activities," the military has said.

The offensive has included air strikes on Boko Haram strongholds in remote parts of northeastern Borno state, and has spread to the state capital Maiduguri, the insurgents' traditional home base -- which residents said Sunday was under a blockade.

The defence ministry in a mission update said 14 insurgents and three soldiers had been killed in battles since Saturday and that Special forces troops were continuing "the advance and attack on identified terrorist camps".

"Patrols are also ongoing to secure towns and villages from infiltration, while curfews on identified flash points are being enforced," the ministry statement said.

Soldiers have sealed roads heading out of Maiduguri, blocking supply routes to remote towns where Boko Haram Islamists have seized power, residents said.

"There is a huge build-up of trucks loaded with essential commodities...

along the Baga road on the way out of Maiduguri to the northern part of the state," said resident Ibrahim Yahaya.

"The drivers said they have been prevented by the military from going northward," he told AFP by email.

The phone network in Borno has all but collapsed since President Goodluck Jonathan on Tuesday declared a state of emergency there and in two neighbouring states, Adamawa and Yobe.

The military on Saturday imposed a round-the-clock curfew in 12 Maiduguri neighbourhoods considered Boko Haram bastions.

The curfew was being enforced on Sunday, with most roads deserted, an AFP journalist reported, while some living in areas not impacted by the curfew also stayed indoors.

"My area is not affected but I have to stay at home with my family," trader Ezekiel Adamu said.

He explained that he was afraid of coming across soldiers, who "seem to have more power with the state of emergency".

Supplies were also running short in the city, where Boko Haram was founded more than a decade ago by the radical cleric Mohammed Yusuf, who was killed in detention in 2009.

The price of basic goods has soared with supply lorries prevented from entering the city. 

"We've been eating without meat since Friday... but there is nothing I can do. This is the challenge of emergency rule," said resident David Olutayo.

Supply shortages and price hikes have also started to bite in the town of Gomboru Ngala, on the border with Cameroon, where some Borno residents have fled to escape the air raids.

"Trucks bringing in goods from Maiduguri have ceased since last week," said resident Grema Babagoni, adding that prices had soared as much as 25 percent.

"If the blockade continues for some time we may completely run out of supplies," he told AFP

Residents in Gomboru Ngala can be reached by phone as the service uses Cameroonian lines.

The town has seen an influx of people fleeing the nearby Marte district, one of the areas where Boko Haram chased out the government and removed Nigerian flags.

Marte has been among the areas targeted by air strikes, residents have told AFP.

A senior rescue official who requested anonymity told AFP that he had could not say what impact the offensive has had on civilians because his staff in Borno and Yobe have been unreachable.

"I have sent emails and texts but have not heard anything yet," the official said.

The operation could prove to be the largest ever against Boko Haram.

A brutal crackdown on the insurgents in 2009, concentrated in Maiduguri, killed more than 800 people and forced the Islamists underground for a year.

Since re-emerging in 2010, they have carried out scores of attacks, including gun raids and suicide bombings.

Many fear that like the 2009 crackdown, the current campaign may fail to crush the group.

Nigeria, Africa's most populous country and top oil producer, has been urged to tackle root causes of the conflict, including acute poverty and excessive government corruption which has helped radicalise many young Muslims in the north.

There is also a risk of high civilian casualties, with Nigeria's military having been accused of massive rights abuses in the past.

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

Jonathan has said that negotiations remain possible amid the sweeping offensive, but the Islamists have so far shown no signs of wanting to talk. (AFP)