Maiduguri: Dozens of Nigerian women who were forced to marry Boko Haram fighters have reportedly been slaughtered by their "husbands" before a battle with troops in the northeast town of Bama.

Five witnesses who recounted the massacres said the Islamist militants feared they would be killed by advancing soldiers or separated from their wives when they fled the town. They killed the women to prevent them from subsequently marrying soldiers or other so-called non-believers, they added.

A Chadian soldier in Gambaru, Nigeria, in February. Niger, Cameroon and Chad have launched a regional military campaign to help Nigeria defeat the Boko Haram insurgency. Credit:Reuters

"The terrorists said they will not allow their wives to be married to infidels," said Sharifatu Bakura, 39, a mother of three.

Nigeria's military along with forces from neighbouring Cameroon, Chad and Niger have claimed huge victories over the insurgents in recent weeks but defenceless civilians still face serious threats.