A Nigerian pastor whose daughter was murdered by Boko Haram for refusing to convert to Islam has said he is "grateful that she didn't change her religion".

Pastor Enoch Mark and his wife Marta were in Chibok, Borno State, the night that 276 girls – including their eldest daughters, Monica and Sarah, were taken from their school in the town last April. Some 50 managed to escape, but 219 were kidnapped, and are believed to remain in captivity.

Describing the night of the abduction in an interview on BBC's Panorama last week, he said: "I just hear a heavy bomb blast that even shook the church. I hear the shout of the girls because I am just about 500 metres away from the school compound."

"I heard how they shouted but I could not help." The pastor and his wife were forced to run into the bush with their five younger children to escape the militants, but their older daughters were taken away.

Marta told the BBC: "When we found out Boko Harm had kidnapped our girls, I was distraught. I've never recovered. I am still in terrible pain."

Several weeks after the abduction, Boko Haram released footage of 136 of the schoolgirls, purporting to show them having converted to Islam. The girls were shown dressed in the full Muslim hijab and reciting the Quran.

Pastor Mark said that was informed that his eldest daughter, Monica, was killed by her captors when she refused to convert to Islam.

"I was told that my daughter refused to change her religion. I was told that they dug a hole and buried her from the neck and stoned her to death," he said.

"To die for the sake of Christ, that's the happiest thing for me. I'm grateful that she didn't change her religion. She trust in God."

Marta added that she believes their daughter "died with dignity".

"Monica is now in heaven because she refused to convert," she said.

Two women who have fled captivity by Boko Haram told the BBC that they witnessed some of the Chibok schoolgirls carrying out orders to kill. One young woman said the girls had been "brainwashed" by the militants and she saw them murder a number of Christians.

"They were Christian men. They [Boko Haram militants] forced the Christians to lie down. Then the girls cut their throats," the woman recalled.

"They [the Chibok girls] told us: 'You women should learn from your husbands because they are giving their blood for the cause. We must also go to war for Allah.'"