Abubakar Shekau, the group's leader, said that the girls would be held until all of its imprisoned militants had been freed.
He added that the girls had been converted to Islam and the video, released Monday, claims to have shown them praying.
The Boko Haram abducted more than 200 girls in northern Nigeria April 14 and threatened to sell them.
Three girls were seen speaking in the video and one of them said the group has not been harmed, the report said.
Two girls said they were Christian and have been converted to Islam.
"These girls, these girls you occupy yourselves with... we have indeed liberated them. These girls have become Muslims," Shekau was quoted as saying in the video.
It was thought the majority of the abducted girls were Christians, although there were a number of Muslims among them.
There was no indication of when or where the video was taken.
The number of girls shown in the video was just under half of the 276 pupils abducted by the terrorists from the northern state of Borno.