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 neighbors 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 100 km (60 miles) from the village of Chibok from where Boko Haram abducted more than 200 secondary schoolgirls 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.