The Nigerian military says its troops backed by fighter jets have invaded Boko Haram's last known stronghold, the Sambisa forest in the northeast of the country, in an effort to finally defeat the armed group.

Armies from Nigeria and neighbours Chad, Niger and Cameroon have in the past two months launched a concerted push to try to crush the group, which has killed thousands and abducted hundreds in its battle to establish an Islamic state.

"The operations especially in forest locations are progressing in defiance of obstacles and land mines emplaced by the terrorists," the Nigerian military said in a statement on Wednesday.

Reports from northern Nigeria are difficult to independently verify since the media and non-governmental organisations have limited access.

But a member of the Civilian Joint Task Force - made up of Nigerians supporting military operations against Boko Haram - confirmed the latest reports to Al Jazeera and added that fighting had left five vigilantes and two soldiers dead.

Chibok girls

The Sambisa forest is about 100km from the village of Chibok from where Boko Haram abducted more than 200 secondary school girls a year ago.

Mohamed Ibn Chambas, UN Special Representative for West Africa, said this month that Boko Haram fighters seeking shelter in the Sambisa Forest may be using some of the captured girls as human shields.

An official in the Chadian army told the Reuters news agency that Chadian and Cameroonian troops were ready to attack Sambisa, which lies near the Cameroon border, from the other side and would move in soon.

Chadian troops were assembling in Mora, northern Cameroon, ahead of the joint operation, a Cameroonian army official said.

Boko Haram controlled an area the size of Belgium at the start of the year, but have since lost much of that ground.

Yet they remain a deadly threat to civilians, as illustrated on Friday when they slit the throats of 12 people in northeast Nigeria as the army was trying to evacuate the area around the former Boko Haram headquarters of Gwoza.

Failure to crush Boko Haram or protect civilians was one reason President Goodluck Jonathan lost an election on March 28 to Muhammadu Buhari, who has pledged to spare no effort in battling the group after he was sworn in on May 29.

Buhari also said he would do everything possible to rescue the Chibok girls, but could not promise to find them.

Source: Al Jazeera And Reuters