Boko Haram militants, hiding in the Sambisa forests, cannot yet be defeated, though cornered by Nigeria’s military.
"Everywhere they have their havens, they have mined it all around," Major General Chris Olukolade, spokesman for Nigeria's defence headquarters, told Reuters in an interview.
Boko Haram expands on a land bigger than Belgium in the northeast of Nigeria and caused a global outcry after abducting over 200 schoolgirls from Chibok, a town in Nigeria, in April 2014.
Nigerian army launched a counterattack in January 2015, after six years of silence when thousands were killed and forced to migrate from their homes.
According to Reuters, the Nigerian army says Boko Haram has now been pushed into the vast Sambisa forest in eastern Borno state.
The group has made no public comment since its last audio clip, in early March, a pledge of allegiance to ISIS that has seized swathes of Syria and Iraq.
Nigeria launched a ground offensive into Sambisa last month following aerial bombardments and says over 13 Boko Haram camps have since been dismantled.
"The Sambisa forest is another place that is seriously mined ... A major obstacle to the move," Olukolade said.
"By intelligence reports, we have many more (militant camps) and they must be destroyed."
Boko Haram has been pushed back before but Olukolade said to Reuters that he hoped the military would be able to hold its gains through aggressive patrols.
Nigeria’s armed forces are joined by those of neighbours Cameroon, Niger and Chad who also combat the militants.
Nearly 600 women and children, held in the Sambisa forest, have been freed by the Nigerian army over the last week.
Amnesty International told Reuters that around 2,000 women and girls had been captured and forced to be sex slaves, fighters and cooks or had been executed in April.