Jihadists aligned to Islamic State have killed one of six aid workers abducted in July in northeast Nigeria, charity group Action Against Hunger said Wednesday.
The six Nigerian aid workers -- one woman and five men -- were seized by jihadists during an ambush on their convoy close to the border with Niger.
"The armed group holding captive an employee of Action Against Hunger (ACF), two drivers and three health ministry personnel, have executed a hostage," the Paris-based organisation said in a statement.
"Action Against Hunger condemns in the strongest terms this assassination and urgently calls for the release of the hostages," it said, without giving details of the identity of the victim.
The charity said it was "extremely concerned and is fully mobilised to ensure that the remaining hostages can be quickly and safely reunited with their families".
The Islamic State West Africa Province (ISWAP) group released a video following the abduction of the female ACF staff member in which she pleads for the release of the hostages with her five male colleagues behind her.
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The kidnapping was the latest to target aid workers in the conflict-hit region after the abduction and killing of two female staff for the International Committee of the Red Cross (ICRC) last year.
United Nations Humanitarian Coordinator in Nigeria Edward Kallon said he was "appalled and deeply saddened by the news of the horrific execution of an aid worker this morning".
"I am also extremely concerned about the increasingly dangerous and restrictive operating environment for implementing humanitarian assistance in crisis-affected areas," he said in a statement.
ISWAP is a splinter faction of jihadist group Boko Haram that swore allegiance in 2016 to IS leader Abu Bakr al-Baghdadi.
It has repeatedly attacked military bases and targeted aid workers in northeast Nigeria.
The announcement of the latest execution comes after the army last week shut down ACF offices in northeast Nigeria, accusing the organisation of supplying "food and drugs" to the jihadists.
A second aid group, Mercy Corps, said Wednesday that it was suspending its operations in the region after the military closed its offices.
"We have not yet received an official reason from the Nigerian authorities for the closure and we are seeking to work with them to resolve this as soon as possible," Mercy Corps said in a statement.