Six of the 136 students kidnapped from an Islamic school in Tegina Niger state have died of illness, the school principal said.
The abductors have demanded a ransom to release the students, kidnapped in May after an armed gang on motorcycles attacked the school.
The principal, Abubakar Garba Alhasan, said the kidnappers had called to say the children died from sickness and to urge that the ransom demand be met.
Abubakar Adam, whose seven children are held by the gang, said the abductors called the principal to demand a ransom.
In Kaduna state, kidnappers on Sunday released 15 more students taken last month from Bethel Baptist High School, after parents paid an undisclosed ransom to free them.