Boko Haram leader Abubakar Shekau directly ordered women to be killed in the northeast Nigerian town of Gwoza, one man who was forcibly conscripted into the militant ranks claimed on Friday.

Usman Ali said he witnessed the killing in the town, which the group's elusive leader proclaimed as part of a caliphate last year and which has generally been seen as the militants' headquarters.

Nigeria's military on Friday announced that troops had retaken the town of Gwoza from Boko Haram, from which the group declared their caliphate last year.

"Troops this morning captured Gwoza destroying the Headquarters of the Terrorists self-styled Caliphate," Defense Headquarters in Abuja said on Twitter.

Hundreds of people who escaped a Boko Haram attack need urgent help, a local lawmaker said on Wednesday, as those still hiding on a mountain a week later said food and water supplies had run out.

Islamist militants attacked Gwoza in Borno state, northeast Nigeria, last Wednesday, leaving dozens of people dead and sending others fleeing for their lives, some to hills near the border with Cameroon.