Boko Haram has released a new video showing more than 100 girls, believed to be those who were abducted in northern Nigeria four weeks ago, Radio New Zealand reports.
More than 200 girls were kidnapped on 14 April from their school in the northeastern village of Chibok.
They were in their final year of school, most of them aged 16 to 18.
Their location is still unknown.
Boko Haram leader Abubakar Shekau said they would be held until all members of the group who are in prison had been freed.
Interior Minister Abba Moro rejected the deal, telling the BBC that it was "absurd" for a "terrorist group" to try to set conditions.
In an earlier video obtained by AFP on 5 May, Boko Haram threatened to sell the girls.
Two girls in the video also said they were Christian and have now converted to Islam.
"These girls, these girls you occupy yourselves with... we have indeed liberated them. These girls have become Muslims," Abubakar Shekau says in the video.
He said his offer to swap the girls for prisoners only referred to the children who had not converted to Islam.
Some 136 girls are shown in the video, just under half of the 276 pupils abducted from their school in the northern state of Borno.