Militants fighting for Boko Haram in Nigeria have released the first images of their terror activities since pledging allegiance to ISIS leader, Abu Bakr al-Baghdadi, the Daily Mail of UK has reported.

Taken somewhere in the forests of north-eastern Nigeria, the images show the jihadis casually posing in front of the terror group's sinister black and white flag while brandishing assault rifles.

The slick photographs carry all the logos and artwork typically seen in official ISIS releases, suggesting the Middle East-based militants have taken full control of Boko Haram propaganda.

The release came as Nigerian soldiers backed by warplanes invaded the Islamist's final stronghold in the country - the Sambisa forest - in an effort to finally defeat the six-year-old insurgency.

Over the last six years, Boko Haram have killed thousands and kidnapped hundreds in a battle to establish an Islamic caliphate in Nigeria.

Earlier this year the group's leader, Abubakar Shekau, released a video saying the group had pledged allegiance to ISIS and would be dropping the name Boko Haram.

Instead they now refer to themselves as Wilayat al Sudan al Gharbi, which is commonly translated into English as the 'West African Province' of the Islamic State.

Since their ISIS allegiance video earlier in the year, Boko Haram has not released any official propaganda photographs or video footage.

Typically ISIS and their affiliates release shocking images of mass executions or the brutal enforcement of Sharia law after similar periods of silence.

The fact Boko Haram have returned with little more than a series of group shots and pastoral scenes is perhaps symptomatic of the group's rapid decline in influence over the past months.

On Thursday, Nigerian soldiers retreated from Boko Haram's last known stronghold in the country amid concerns the militants had booby-trapped the area before fleeing.

Three pro-government vigilantes were killed in the area by a landmine on Thursday morning, according to Daily Mail.

A vigilante and a security source both confirmed the pullback from the Sambisa forest, a day after an offensive aimed at rooting out the insurgents.

A spokesman for the military was not immediately available for comment.

"The soldiers have retreated to Bama because of mines. They had been on the road but that made them vulnerable, so they moved to the bush but there are mines planted there (too)," one soldier, who did not want to be named, revealed.

The Sambisa forest, a former colonial game reserve, is about 60 miles from the village of Chibok, from where Boko Haram abducted more than 200 secondary school girls a year ago.

"Three of our boys were killed by a landmine as we progressed into Sambisa. We've suspended going farther," Muhammad Mungonu, a member of a  vigilance group, told Reuters.

Intelligence officials believed the girls kidnapped in Chibok are being held in the Sambisa forest, but US reconnaissance drones have so far failed to locate them.

Boko Haram controlled an area the size of Belgium at the start of the year, but have since lost much of that ground after a concerted push by militaries from Nigeria and neighbours Chad, Niger and Cameroon in the past two months.

Yet they remain a deadly threat to civilians, as illustrated last Friday when they slit the throats of 12 people in northeast Nigeria as the army was trying to evacuate the area around the former Boko Haram headquarters of Gwoza.