Christian Solidarity Worldwide (CSW) has welcomed assurances that efforts are underway to secure the release of Leah Sharibu.
The Christian teenager was one of 110 schoolgirls kidnapped from their school in Dapchi, Yobe state in Nigeria, nearly four years ago by the Islamic State West Africa Province (ISWAP).
The others were released shortly after but the captors told Leah she would continue to be held after she refused to convert to Islam.
ISWAP said in October 2018 that Leah had been made a "slave for life", and she remains in captivity with little known about her welfare.
CSW said action to secure her release was "long overdue" and called on the Nigerian government to do more to rescue her.
Nigeria's Chief of Defence Staff, General Lucky Irabor, told the Good Morning Nigeria TV show on Tuesday that he was "aware of plans" to try and free her from captivity.
There were, he said, "processes that are in place to ensure that not just Leah Sharibu but every other person held captive is released."
He also revealed that another Chibok schoolgirl was rescued a few weeks ago. The 276 Chibok girls, who were mostly Christian, were taken by terrorist group Boko Haram in 2014. Over a hundred are still in captivity.
CSW's Founder President Mervyn Thomas said: "CSW welcomes General Irabor's assurance that efforts are underway to rescue Leah Sharibu and other captives.
"Action is long overdue, so we urge the Nigerian government to spare no effort in following through on these commitments."