The 200 Chibok schoolgirls kidnapped by Boko Haram last year are likely being held inside bunkers in the Sambisa Forest in Nigeria, Borno State governor Kassim Shettima has revealed.

"We are suspecting that Chibok girls are living with the insurgents in bunkers, I think the military must carry out their operations beyond the surface earth," Shettima said on Tuesday, according to Nigeria's Daily Post.

"They (Boko Haram) were also known to have dug tunnels to enable them move from house to house. So, having been left unchallenged for such a long time, such possibility cannot be ruled out, which poses serious obstacles within the forest," he added.

Last April, the schoolgirls were kidnapped from the town of Chibok by the extremist group, which has terrorized the northeastern regions of Nigeria for six years in an attempt to establish an Islamic caliphate. 

Since then, hope that they will be found has steadily dwindled, despite repeated promises from former Nigerian President Goodluck Jonathan.

Although several dozen girls managed to escape as the kidnappers were taking the hostages to the Sambisa Forest in northeast Nigeria, 219 remain missing. While the exact nature of their fate is unknown, Boko Haram's leader, Abubakar Shekau, has claimed that schoolgirls, many of them raised as Christians, have been converted to Islam and "married off" to Boko Haram fighters or sold for as little as $12.

One of Boko Haram's former captives, Liatu Andrawus, a 23-year-old mother of two children, told The Globe and Mail that she met the hundreds of captured schoolgirls while she was being held in captivity in the northeastern Nigerian town of Gwoza.

She recalled how she and the girls would gather to pray and comfort each other, and discuss how to flee the militant group.

"We always talked about how we could escape. Sometimes we sat down and prayed together and hugged and cried. They were remembering their good moments with their parents and loved ones," Andrawus said.

She also revealed that some of the schoolgirls she met were forcibly married to Boko Haram fighters, while others were kept hostage in a compound.

Last year, the kidnappings sparked international outrage and the hashtag #BringBackOurGirls. This year, activists marked the anniversary with a change in their slogan from "Bring Back Our Girls - Now and Alive" to "Never to be Forgotten."

At least 2,000 women and girls have been abducted by Boko Haram since the start of 2014, and many have been forced into sexual slavery and trained to fight, Amnesty International has revealed.

"They married me," said Hamsatu, 25, who was recently rescued by the Nigerian army along with hundreds of others. She said she was four months pregnant, that the father was a Boko Haram member and that she had been forced to have sex with other militants who took control of her town.

"They chose the ones they wanted to marry," added Hamsatu, whose full name was not used to protect her identity. "If anybody shouts, they said they would shoot them."

Hundreds of boys and young men also have been kidnapped and forced to fight with the extremists, or slaughtered for refusing to do so, it said. Boko Haram, whose attacks on schools have forced thousands out of education, translates as "Western education is forbidden."

Speaking on Thursday, the governor also claimed that the death toll from Boko Haram insurgency in Nigeria is much higher than reported.

"This year alone, more than 70,000 people were killed before Boko Haram was knocked out. So far, more than 300,000 people have been killed. I lost 29 of my siblings. I buried them myself," Shettima said.