Nigerian forces backed by warplanes invaded Islamist group Boko Haram’s last known stronghold, the Sambisa forest, on Wednesday, in an effort to finally defeat their six-year-old insurgency, two military sources said.

Armies from Nigeria and neighbours Chad, Niger and Cameroon have in the past two months launched a concerted push to try to crush the insurgents, who have killed thousands and kidnapped hundreds in their battle to establish an Islamic state.

The Sambisa forest in northeast Nigeria, a vast former colonial game reserve, is about 100km (60 miles) from the village of Chibok from where Boko Haram abducted more than 200 secondary school girls a year ago. Intelligence officials had believed that this was where they were being held, although U.S. reconnaissance drones have failed to find them.

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

A spokesman for the military was not immediately available for comment.

An official in the Chadian army said 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.

•Excerpted from a Reuters report. Photo shows Nigerian attack helicopter in operation against Boko Haram insurgents.