Kidnapped Dapchi schoolgirl Leah Sharibu, who has been held by the Boko Haram terror group since February, has pleaded for her freedom in newly-released audio.

Sharibu, 15, is being held by a faction of Boko Haram after she reportedly refused to renounce her Christian faith.

Most of the students abducted from their boarding school in Dapchi village, Yobe State, February 19, were released after four weeks.

"I am Leah Sharibu, the girl that was abducted from Government Girls Science Technical College, Dapchi. I am calling on government and people of goodwill to get me out of this problem," she says in the audio in her native Hausa language.

"I am begging you to treat me with compassion, I am calling on the government, particularly, the president, to pity me and get me out of this serious situation."

Her father, Sharibu Nathan confirmed to CNN that it was his daughter speaking on the audio. He added that he was happy to hear her voice.

"I thought she might have been killed since we were told by those released that Boko Haram kept her because she is a Christian. I can only imagine the way they would have treated her," Nathan said.

Nathan said he is hopeful that the recording, obtained by a local journalist, will redouble efforts to free her.

"I have been calling the government to save my daughter. It has been seven months since she was taken, I believe they can get her from Boko Haram if they want to help us," Nathan said.

President Buhari's spokesman Garba Shehu said the secret service was analyzing the voice recording and government reaction will be made known after the investigation.

Shehu said Buhari will not rest until Sharibu and other girls in the group's captivity are reunited with their parents.

"For President Buhari, nothing will be spared in bringing all our girls home. He will not rest until all of them are freed," Shehu said in a tweet.

Boko Haram has kidnapped more than 1,000 childrenin Nigeria since 2013.

The group, however, gained global notoriety following the mass kidnapping of more than 200 girls from a boarding school in Chibok town, Borno State in April 2014.

So far 93 of the Chibok girls were freed in a swap deal between the Nigerian government and the insurgent group.

More than 100 of them remain in captivity with their whereabouts unknown.