Nigerian troops with warplanes attacked the last known hideout of the Boko Haram terror sect, the Sambisa forest, on Wednesday, April 22.
The purpose of the invasion is to rescue the Chibok girls, who were abducted a year ago and to finally overcome Boko Haram's six-year-old terrorism, Reuters said.
Soldiers from Nigeria and neighbouring countries Chad, Niger and Cameroon have in the past two months launched a combined push to try to defeat the terrorists, who have killed thousands and abducted hundreds in their battle to launch an Islamic state.
The Sambisa forest in northeast of Nigeria is about 100 km (60 miles) from the village of Chibok from where Boko Haram terrorists kidnapped more than 200 secondary schoolgirls on April 14, 2014.
Security officials believe that this was where they are being held, although US intelligence drones failed to find them. A spokesman for the army is yet to confirm the information.
An official in the Chadian military said joint Chadian and Cameroonian armies were ready to invade Sambisa, which lies on the Cameroon border, from the other side and would move in soon.
The insurgents controlled an area the size of a European country at the beginning of the year, but have since lost much of that ground after the shift of the general elections on February 7.
They still remain a deadly threat to Nigerians, as showed on Friday when they beheaded 12 people in the northeast of the country, as the army was trying to empty the area around the former Boko Haram headquarters of Gwoza.
His o defeat Boko Haram or protect civilians was one of the reasons for the failure of President Goodluck Jonathan in the presidential poll on March 28.
The current leader lost to the former military ruler Muhammadu Buhari, who has vowed to spare no effort in fighting the terrorists after he is 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.