In a statement, defence spokesman Brigadier-General Chris Olukolade also said 23 women and 35 children being held on charges of aiding Islamist militant group Boko Haram had been released as a gesture of peace to its more moderate sympathisers.

Nigerian forces are carrying out their most concerted effort yet to end a four-year insurgency that has left thousands dead – many of them killed in gun or bomb attacks – and destablised swathes of the north of Africa’s top oil producer.

“The mandate to the forces involves the destruction of all terrorist camps and apprehension of perpetrators,” Olukolade said. “This mandate has been substantially achieved with destruction of terrorists’ strongholds. A number of terrorists have been apprehended … Many of them have died in battle.”

Boko Haram, which is fighting for an Islamic state in religiously-mixed Nigeria, have in the past proved masters of resurrecting themselves after apparent defeat.

Nigerian authorities thought they were finished after a 2009 crackdown left 800 dead, including the sect’s founder Mohammed Yusuf, but they came back stronger than ever, developing ties with al-Qaeda linked militants in the Sahara.

President Goodluck Jonathan declared a state of emergency on May 14, ordering extra troops into the northeast after reports Boko Haram had taken over large stretches of the remote semi-desert region.

Since then, security sources say the number of attacks has dropped sharply, although a spate of deadly attacks on schools showed the sect can still inflict mayhem.

Olukolade said the destruction of bases in Nigeria had pushed the militants back inside northeast Nigeria’s cities, like Maiduguri, where many had been seized in cordon and search operations.

On Sunday, Nigerian forces said they had uncovered a vast network of underground tunnels connecting houses and bunkers in the Maiduguri area of Bulabulin, as well as some collective graves of people killed by the militants. Olukolade said some of the bunkers could accommodate over 100 people.

Critics say no amount of military force will solve the underlying issues driving the insurgency, such as the north’s poverty and sense of political exclusion, but efforts to dialogue with the sect have failed to get a positive response.

The authorities had also arrested the parents-in-law of Boko Haram leader Abubakar Shekau. On Saturday Shekau uploaded a defiant Internet video vowing never to “dialogue with a government that is corrupt and using the book of pagans (Bible) to run itself,” a swipe at the fact that Jonathan’s administration is dominated by southern Christians like himself. DM

Photo: Soldiers from Lagos, part of an expected 1,000 reinforcements sent to Adamawa state to fight Boko Haram Islamists, walk near trucks as they arrive with the 23rd Armoured Brigade in Yola May 20, 2013. REUTERS/Stringer