MINNA, Nigeria (Reuters) - Six of the 136 students kidnapped from an Islamic school in the north-central Nigerian state of Niger have died of illness, the school principal told Reuters on Monday.

       The abductors have demanded a ransom to release the students, kidnapped in May after an armed gang on motorcycles attacked the school in the town of Tegina.

       Criminal gangs carrying out kidnappings for ransom are blamed for a series of raids on boarding schools in northern Nigeria in which more than 1,000 students have been abducted since December.

       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.

       Kidnappers on Sunday released 15 more students taken last month from a Baptist school in northwest Nigeria, after parents paid an undisclosed ransom to free them.

       President Muhammadu Buhari in February called on state governments to stop paying kidnappers, and Kaduna Governor Nasir El-Rufai publicly refuses to pay. Desperate parents and communities often raise and pay ransoms themselves.

       (Reporting by Maiduguri newsroom; Writing by Chijioke Ohuocha; Editing by Peter Cooney)