A flag belonging to Boko Haram flies from a atop a mosque in the town of Damasak, Nigeria, 18 March 2015 Reuters

Nigerian terror group Boko Haram's last known stronghold has been invaded in an offensive by the Nigerian army backed by neighbouring countries.

The attack occurred in the Sambisa forest, on the border with Cameroon, where the insurgents have recently expanded their activity.

Who is Boko Haram?

Boko Haram fights against Western influence in Nigeria and aims to impose its version of Sharia law in the country. The group declared an Islamic caliphate in Gwoza, along the Cameroon border, in August 2014.

Boko Haram has raided several cities in the north of the country in a bid to take control of more land.

Three states, Adamawa, Borno and Yobe, have been under a state of emergency since May 2013, due to Boko Haram's deadly attacks.

A Chadian army official told Reuters that allied Chadian and Cameroonian troops were ready to attack Sambisa from the other side.

Intelligence officials believe that Sambisa is the location where more than 200 schoolgirls were kept captive after being abducted by Boko Haram in April 2014.

Originally around 270 schoolgirls from Chibok, a village in Borno state, were kidnapped by the terrorists. Shortly after, some 50 girls managed to escape but the rest are still missing amid reports they are being raped, forced to marry their abductors and used as suicide bombers.

Former president Goodluck Jonathan, who lost the 2014 general election to former military chief Muhammadu Buhari, was often accused of not having done enough to find the girls and halt the terrorists, who have killed thousands of people since their insurgence started in 2009.

Jonathan recently told the UN that Nigeria does not need the help of foreign countries to defeat the terrorists and that the international community should focus on providing aid to the thousands of people displaced due to the insurgence. As per UN's latest estimates, at least 1.5 million people, of whom 800,000 are children, have fled their homes to escape violence.

Once he became Nigeria's president, Buhari vowed to defeat Boko Haram, which recently pledged allegiance to Islamic State (Isis) terror group. Buhari also promised his country would do everything to bring the Chibok girls home.

The Nigerian military is being aided by troops from Chad, Cameroon, Benin and Niger. Before starting the Sambisa offensive, it had managed to push the insurgents out from several areas they previously controlled.