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

An offensive by Nigerian troops against terror group Boko Haram in the Sambisa forest has been temporarily halted after mines placed by the insurgents killed three people.

The incident occurred shortly after the Nigerian army announced that its offensive had reached Boko Haram's last known stronghold.

Intelligence officials believe that Sambisa – in Borno state, on the border with Cameroon – is the location where more than 200 schoolgirls are being held 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 afterwards, 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.

Boko Haram Nigeria

Who are 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 (pic: Reuters)

A vigilante who took part in the offensive told AFP: "Boko Haram have buried landmines all over the routes leading to their camps in the forest, which is no doubt a huge obstacle regarding the military offensive against them.

"We decided to turn back since the route was unsafe. As we were driving back, one of the vehicles carrying CJTF [Civilian Joint Task Force] hit a mine.

"A soldier and three CJTF were killed while another soldier was injured. We trudged along and made it back to Bama yesterday [22 April]."

The operation is expected to resume.

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.

The offensive was launched before Nigeria held president elections in which Goodluck Jonathan was defeated by former military chief Muhammadu Buhari.

Jonathan was often accused of not doing enough to find the girls and halt the terrorists, who have killed thousands of people since their insurgence started in 2009.

He recently told the UN that Nigeria does not need the help of foreign countries to defeat Boko Haram and that the international community should focus on providing aid to the thousands of people displaced due to the insurgence.

As per the 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.