Traditional African rulers should apologise for the role they played in the slave trade, a Nigerian rights group has said in a letter to chiefs.

The letter said some collaborated or actively sold off their subjects.

The group said it was time for African leaders to copy the US and the UK who have already said they were sorry.

It urged Nigeria's traditional rulers to apologise on behalf of their forefathers and "put a final seal to the history of slave trade", AFP news agency reports.