By Ardo Abdallah
BAUCHI, Nigeria (Reuters) - Nigerian soldiers retreated from Islamist group Boko Haram's last known stronghold in the country's northeast on Thursday, concerned the area was booby-trapped after three pro-government vigilantes were killed by a landmine.
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.
"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, told Reuters.
The Sambisa forest, a former colonial game reserve, is about 100 km (60 miles) from the village of Chibok, from where Boko Haram abducted more than 200 secondary school girls a year ago.
Intelligence officials believed they were being held in the forest, but U.S. reconnaissance drones failed to locate them.
"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 pro-government vigilante, told Reuters.
The militants 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 neighbors Chad, Niger and Cameroon in the past two months.
Chadian military source said a joint military operation involving armies from Niger and Cameroon was expected to begin to encircle the Sambisa forest next week. Chadians will go in from the Cameroonian border where they have been massing troops.
Over the last six years, Boko Haram have killed thousands and kidnapped hundreds in a battle to establish an Islamic caliphate, a factor that cost President Goodluck Jonathan victory in Nigeria's March 28 election.
(Additional reporting by Madjiasra Nako in N'Djamena; Writing by Helen Nyambura-Mwaura; Editing by Tim Cocks and Crispian Balmer)