A Nigerian government official has addressed rumours that abducted Christian schoolgirl Leah Sharibu has been killed. 

A statement released by the special assistant to President Muhammadu Buhari said that the 16-year-old, abucted from her school by militants last year, is still alive. 

The statement said that the government was continuing negotiations with the Islamic State West African Province (ISWAP), the faction of the Islamist terrorist group Boko Haram that is holding Leah. 

"Lines of communications remain open with the kidnappers, ISWAP, to secure the release of Leah Sharibu," the statement reads.  

"Contrary to false reports, she is alive – [we have been] given assurances by our security agencies – and the government is committed to her safe return, as well as all other hostages to their families."

It added that the government had no intention of giving in to demands for a ransom. 

"Kidnapping for ransom should never be encouraged. This means not capitulating to the demands of terrorists and refraining from rewarding their heinous crimes with payment," the statement said.

Leah was 14-years-old when she was abducted along with over 100 of her classmates from her secondary school in Dapchi, Yobe state, on 19 February 2018.

The other girls were all released several weeks later but the militants did not let Leah go because she refused to renounce her Christian faith. 

In October last year, ISWAP said they had decided to retain her as a slave for life. 

Dr Matthew Rees, Head of Advocacy at Open Doors UK and Ireland, said: "Leah Sharibu was kidnapped because she is a girl and held captive because she is a Christian.

"She embodies the incredibly vulnerable position of Christian women in northern Nigeria.

"It is outrageous that Leah remains in captivity, abused as a PR tool and negotiating pawn by Boko Haram. However, the government's assurance that it will make more efforts to secure her release is encouraging."

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