The police in Nigeria have freed 500 students from an Islamic school in the north of the country, some of whom were in chained up, in the third such incident in less than a month, Reuters news agency reports quoting police sources.

This comes two days after 67 people were found shackled in another school in Katsina state.

Last month, more than 300 male students were freed from a similar boarding school in neighbouring Kaduna state.

Reuters says two witnesses at the scene said the school's owner and five staff members have been arrested.

The police have made no official comment