The purported head of Nigerian Islamist group Boko Haram has claimed two recent deadly attacks in the northeast in a video obtained by AFP Monday, which also depicts women and children apparently being held hostage.

The video features Abubakar Shekau, declared a global terrorist by the United States, seated on a rug with a kalashnikov resting behind his right shoulder.

"We are the ones that carried out the Bama attack," Shekau said in the Hausa language, referring to the 7 May  assault that killed 55 people, mostly soldiers and police.

"We also carried out the attack in Baga," he further said of the 16 April raid in the town near Lake Chad that sparked clashes with soldiers which killed nearly 200.

Nigeria's military has been accused of causing scores of deaths in the Baga violence by deliberated setting fires that razed thousands of homes.

"It was you, the security agents that went into town the following day and burnt homes and killed people at will," Shekau said.

The military has fiercely denied the reported abuses by its soldiers, insisting that only 37 people died in the Baga violence, including 30 suspected Islamists.

Some seven minutes into the 12-minute video message, the screen splits, showing Shekau on the left with a group of unidentified women and children on the right.

The Islamist leader claimed this group is being held hostage in retaliation for the wives and children of Boko Haram members detained by the military.

"As long as we do not see our women and children we will never release these women and children," Shekau said.

The insurgents have repeatedly accused the military of arresting their family members.

The images of Shekau resemble those previously released by the group and the message was distributed by email, in a manner consistent with earlier Boko Haram messages.