Abuja - Nigeria's military is confident it has Boko Haram cornered, but a final push to clear the Islamist militants from their forest hideouts is being hampered by landmines, it said on Saturday.
"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 grabbed a swathe of the northeast and caused a global outcry when it abducted over 200 schoolgirls from Chibok.
A counter-attack was launched in January after a six-year insurgency that killed thousands and forced more than 1.5 million people from their homes.
The army says Boko Haram has now been pushed into the vast Sambisa forest in Borno state. The group has made no public comment since its last audio clip, in early March, a pledge of allegiance to the Islamic State group that has seized swathes of Syria and Iraq.
The militant group was not available for comment. It communicates erratically via hard to verify videos posted online or sent to a few journalists.
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 he hoped the military would be able to hold its gains through aggressive patrols and now that Cameroon, Niger and Chad were combatting the militants.
"This tendency to escape into safe havens out of Nigeria is becoming very remote. The likelihood of them surviving this assault and coming back is low," Olukolade said.
Unlike in recent months when troops from Niger and Chad were operating inside Nigeria to help the counter-insurgency, Olukolade said there was no significant presence of foreign soldiers on Nigerian soil any more and only the Nigerian military was involved in the Sambisa offensive.