The father of a Nigerian schoolgirl being held by Islamist militant group since February has told the BBC he is delighted to hear her voice in a recently released audio recording.

"It really is her voice. Before, I thought she wasn't even alive," Nathan Sharibu told BBC Pidgin.

His daughter was kidnapped along with more than 100 girls from Dapchi town.

The others were later freed but she was kept, reportedly because she refused to convert from Christianity to Islam.

The 35-second audio recording, which is yet to be authenticated by Nigerian authorities, was published by Nigeria's cable newspaper on Monday.

In it, the 15-year-old girl calls on the government to help her.

Her father said the recording had given the family hope: "I'm really encouraged that I heard the words of my favourite daughter."

Mr Sharibu said the whole family was pleading for the government to do more - as were "Christians and some Muslims that are praying for my daughter".

"We are very, very sad in the family. I hope they will put more pressure concerning her release." (BBC)