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 massacressaid 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.

"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.

According to Ms Bakura's account, which was supported by others, Boko Haram fighters received word of a military assault on Bama, formerly an Islamist stronghold in Borno state.

The insurgents had decided to flee to the nearby town of Gwoza before the troops' arrival but first decided "to kill their wives so that nobody will remarry them", she said.

Ms Bakura's husband was killed by the insurgents four months ago but she was spared from a forced marriage because she was visibly pregnant.

Boko Haram forcibly married scores of women in Bama after seizing it in September. Nigeria's military announced the recapture of the town on Monday.