More than 50 of the girls abducted by Boko Haram militants in Nigeria last year were seen alive as recently as three weeks ago, a Nigerian woman has told the BBC.

The woman—who did not want to be identified out of fear for her safety—told the BBC she saw the girls in northeastern Gwoza before government forces drove Boko Haram Islamic extremists out of the town.

Boko Haram sparked global outrage after seizing at least 276 girls from Chibok town in Nigeria last April. Dozens were able to escape but at least 219 remain missing. Boko Haram leader Abubakar Shekau has said the girls have been converted to Islam and married off. Relatives of the girls feared the militants had used them as bartering goods and sex-slaves.

Shekau has pledged allegiance to the Islamic State (ISIS), which is known for carrying out abductions in Syria and Iraq.

Despite international pleas for their return, the girls have never been traced, and little has been heard of them since they were taken from dormitories during a nighttime raid at their Chibok boarding school.

The unidentified woman who spoke to the BBC, said she lived under Boko Haram's rule in Gwoza, and saw the girls in Islamic attire, being escorted by militants.

"They said they were Chibok girls kept in a big house," the woman said. "We just happened to be on the same road with them," she added.

Three other female witnesses also told the BBC they had seen the girls in Gwoza. Boko Haram was believed to have turned Gwoza into its headquarters after it captured the town in August 2014.

Nigeria's military-- backed by troops from neighboring countries-- recaptured the town in March. Many of the Islamic militants were suspected to have fled to the nearby Mandara Mountains, near the border with Cameroon. But whether the girls went with them is unclear.  

Another woman told the BBC she last saw some of the girls—who range in age from 16 to 18-- in November at a Boko Haram camp in Bita village, also in the northeast.

"About a week after they were brought to the camp, one of us peeked through a window and asked: 'Are you really the Chibok girls?' and they said: 'Yes'. We believed them and didn't ask them again," the woman said.

"They took Koranic lessons, cleaned their compound, cooked for themselves and they braided each other’s hair. They were treated differently - their food [was] better and water clean," the woman added.

Nigeria's outgoing President Goodluck Jonathan has been under fire for not doing enough to find the girls and end the six-year insurgency in the northeast. Incoming President Muhammadu Buhari has promised to "crush" the insurgents. He is expected to be inaugurated on May 29 after defeating Jonathan in last month's presidential elections.

Boko Haram means “Western education is forbidden" in the Hausa language.  The terror group has killed thousands and displaced some 800,000 children, according to a UNICEF report released Monday.  

Nobel Peace Prize winner Malala Yousafzai criticized Nigeria’s government and the international community and called for more action to free the Chibok schoolgirls in an open letter to the girls Monday.  

“We cannot imagine the full extent of the horrors you have endured. But please know this: we will never forget you," Yousafzai said in the letter.

“I look forward to the day I can hug each one of you, pray with you, and celebrate your freedom with your families," Yousafzai said.

The campaign group Bring Back Our Girls has launched a week of events in Nigeria encouraging people to remember the girls ahead of the first anniversary of their abduction on Tuesday.