Two bomb blasts ripped through the outskirts of Nigeria’s capital Abuja on Friday, including one target hit twice before by Boko Haram, after separate strikes in the northeast that killed at least 21.
Nigeria’s National Emergency Management Agency (Nema) warned of casualties from the simultaneous explosions in Kuje and Nyanya and likened the explosives used to those in areas worst-hit by the six-year insurgency.
The 28-minute message, which cannot be independently authenticated by CNN, was posted online by ISIS supporters.
The group's leader, Abubakar Shekau, said they would be held until all imprisoned militants had been freed.
He said the girls had converted to Islam. The video, released on Monday, claims to show them praying.
Boko Haram abducted more than 200 girls in northern Nigeria on 14 April and threatened to sell them.
The BBC's John Simpson in the northern city of Maiduguri said Boko Haram's comments showed signs that the group was willing to negotiate.