Nigerian forces backed by warplanes invaded Boko Haram's last known stronghold, the Sambisa forest, on Wednesday, in an effort to finally defeat their six-year-old insurgency, according to The BBC.

The Sambisa forest in northeast Nigeria is about 100 km (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 US reconnaissance drones failed to find them.