Kidnappers are demanding a ransom of one million naira each to release around 80 children snatched from a boarding school in northern Nigeria last month, according to a pastor involved in the negotiations for their release.

The attack on the Bethel Baptist High School in the state of Kaduna was the 10th mass school kidnapping since December in northwest Nigeria, which authorities have attributed to criminal gangs seeking ransom payments.

"[Bandits] are asking for 1m naira on each of the 80 students remaining with them," Reverend Ite Joseph Hayab told Reuters by telephone.

Kidnappers released 28 children last month after the first batch of 28 was released two days after the raid. But another 81 remain in captivity.

Hayab said three students escaped before the 28 were released last month but they were kidnapped again by an unidentified person in the forest who demanded a ransom and was paid over 1m naira by parents.

Nigerian authorities have attributed the kidnappings to what they call armed bandits seeking ransom payments.

Schools have become targets for mass kidnappings for ransom in northern Nigeria by armed groups.

Such kidnappings in Nigeria were first carried out by the militant group Boko Haram, and later its offshoot Islamic State West Africa Province, but the tactic has now been adopted by other criminal gangs.