Another 11 girls were taken on Sunday from villages near the militants` hideout in a nearby forest.
A team of US experts has been sent to Nigeria to help in the hunt
Earlier this week, the Boko Haram group admitted that its fighters had abducted the girls in the middle of the night from their school in the town of Chibok on 14 April.
Its leader, Abubakar Shekau, threatened to "sell" the students, saying they should not have been in school in the first place, but rather should get married.
The group, whose name means "Western education is forbidden" in the local Hausa language, has staged a wave of attacks in northern Nigeria in recent years, with an estimated 1,500 killed in the violence and subsequent security crackdown this year alone.
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