Science student Sarah Lawan, 19, told The Associated Press that more of the girls could have escaped but that they were frightened by their captors' threats to shoot them.
Lawan spoke in the Hausa language in a phone interview from Chibok, her home and the site of the mass abduction in northeast Nigeria. The failure to rescue 276 of the students who remain captive four weeks later has attracted mounting national and international outrage.
Police say 53 students had escaped and captors are threatening to sell the students still held into slavery.
Lawan spoke as more experts are expected in Nigeria to help in the search, including US hostage negotiators. Nigeria's government belatedly accepted offers of help last week from the United States, Britain, France, China and Spain.
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Also today, a leading Nigerian rights group demanded the UN Security Council impose sanctions on the Boko Haram terrorist network who abducted the girls, saying expressions of concern and condemnation are not enough.
He said targeted sanctions would send a strong message.