An international aid agency says insurgents who kidnapped six humanitarian workers in northeast Nigeria in July claim to have killed four of them.
Action Against Hunger said Friday the latest claim brings the number of fatalities to five, with the first hostage killed in September.
The aid agency called for the "immediate release of our staff member, Grace, who remains in captivity."
The agency said in a statement that in July "an employee of Action Against Hunger, two drivers, and three health ministry personnel were kidnapped while delivering humanitarian aid to extremely vulnerable people in Borno State."
"Action Against Hunger condemns these latest killings in the strongest terms and deeply regrets that calls for the release of the hostages have not been acted upon," the group said.
The militants are believed to be members of the Islamic State in West Africa Province, a splinter group from Boko Haram.
Action Against Hunger says it is currently providing food assistance every month to approximately 300,000 people in northeast Nigeria who have no access to livelihoods and food. In addition, the group says its teams are reaching thousands more with lifesaving health and nutrition services.