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 from northern Nigeria on April 14 and threatened to sell them.
The BBC’s John Simpson in the northern city of Maiduguri said that Boko Haram’s comments showed signs that the group were willing to negotiate.
Three of the girls are shown speaking in the 17-minute video, obtained by French news agency AFP, wearing the full-length hijab.
Two girls say they were Christian and have converted to Islam, while the other says she is Muslim.
Correspondents said the girls appeared calm and one said that they had not been harmed.
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