Leah had in an audio called on President Muhammadu Buhari to save her from captivity.
The 119 girls who were captured were subsequently released, except five who reportedly died in detention, but Leah was held back for “refusing to renounce her faith.”
When asked to give a time-frame for Leah’s rescue, Adesina, who appeared on a Channels TV programme on Tuesday, said God was the sole determinant of her return.
“For how much longer, I think that question can only be answered by God. I believe that God is interested in that young girl and will ensure that she is preserved,” Adesina said.
“I think when that clip that we have is verified, at least one will be sure that it’s her and when that is determined, we should all be glad that she is alive.
“When will she return? By the grace of God, the government is working on it and we believe she will be back.”
Adesina said Buhari had made a commitment to do anything to rescue the kidnapped girls even if it means paying for their release.
The presidential spokesman said: “One thing we should always remember, as Nigerians, is that early in the life of this administration, then it was Chibok girls, the president said that whatever it will take to get the girls back including paying, he would do. It’s on record.”
He, however, denied reports that the government paid a ransom for the release of the other 110 girls.