A year after the abduction of Nigerian Christian schoolgirl Leah Sharibu, her mother Rebecca renewed her appeal to the government to keep its promise to rescue her.

Leah, 15, was abducted on February 19 with more than 100 of her classmates. While the others were released within a month, Leah, the only Christian in the group, remains in captivity.

Islamic State West African Province (ISWAP), a breakaway faction of Boko Haram, announced last Octoberit would keep Leah Sharibu as a slave for life along with Alice Ngaddah, a Christian mother of two who works for Unicef. 'From today, Sharibu, 15, and Ngaddah are now our slaves. Based on our doctrines, it is now lawful for us to do whatever we want to do with them.'

President Mohammadu Buhari pledged to secure her release in a tweet in which he said: 'The thoughts & prayers of all Nigerians are with the Sharibu family, & the families of all those still in captivity. We will do everything we can to bring them back.'

Speaking at a press conference in the capital Abuja last week, Rebecca Sharibu was in tears as she appealed to the government to act.

Open Doors' head of advocacy Zoe Smith said: 'Leah Sharibu was kidnapped because she was a girl and held captive because she was a Christian. She personifies the incredibly vulnerable position of Christian women in northern Nigeria.

'It is saddening and outrageous that Leah remains in captivity, abused as a PR tool and negotiating pawn by Boko Haram. We urge the Nigerian government and the international community to increase their efforts to secure her release and reunite her safely with her family.'

According to reports from her schoolmates, her captors had demanded that she convert to Islam before they would release her but she refused.

Nigeria is number 12 on the 2019 World Watch List, Open Doors' annual ranking of the 50 countries where Christians face the most extreme persecution.