A pastor supporting the family of a Christian teenager kidnapped and held captive by militants in Nigeria has received reports that she is still alive.

Leah Sharibu was abducted by Boko Haram nearly four years ago along with other girls from her school in Yobe State. She was 15 at the time.

While the other girls were freed, Sharibu remains in captivity after refusing to renounce her Christian faith.

Little has been known about her wellbeing in captivity, although there have been reports that she was forced to convert to Islam and marry a militant, and has since had his child.

Rev Gideon Para-Mallam told the Christian anti-persecution charity Open Doors that he had received reports that she is alive.

"From three sources I can confirm that Leah is still alive," he said.

"I can't go into too many details but one of my sources who was recently released from Boko Haram captivity, confirmed that Leah was still alive. She was able to see her."

He said he was not giving up hope that the teenager will eventually be freed.

"It has been almost four years since Leah's abduction. What a traumatic experience for this teenage girl. But we are grateful to God that she is still alive.

"The news of her being alive should encourage our hearts to remain hopeful that one day Leah will be set free."

Nigeria is number 9 on Open Doors' World Watch List, an annual ranking of the 50 countries where Christians face the most extreme persecution.

Open Doors is among the Christian campaign groups calling on the Nigerian government to help secure the release of Sharibu and bring stability to the north of the country where Boko Haram and other militant groups have been wreaking havoc.

In addition to attacks, school children continue to be at risk of abduction, with an estimated 1,000 children forcibly taken from their schools in north-western Nigeria so far this year.

The threat of abductions has caused over 600 schools in the country to close.

"We are distressed and disturbed because the kidnappings continue," Pastor Para-Mallam continued.

"Many are discouraged and feel desolate. Let us pray that God will touch our political leaders so they will be able to see the challenges that are before us."