The aid group Action Against Hunger says one of six hostages being held in north-east Nigeria has been killed.
The BBC understands the so-called Islamic State West Africa Province (Iswap), a Boko Haram splinter group, has been holding the aid workers since July, when a convoy was attacked.
The aid group says one of its employees, two drivers and three health ministry personnel were being held hostage but did not say which one had been killed.
Action Against Hunger has condemned the killing and has urgently called for the release of the other hostages who it says were in the north-east of Nigeria to help the most vulnerable.
Last week the Nigerian army stopped the work of Action Against Hunger – accusing it of helping the militant group Boko Haram. Action Against Hunger denied the accusation.
Earlier this week, Iswap said it had killed two Nigerian soldiers who it had been holding hostage. The authorities have not commented on the Iswap claim.