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.

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.

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.

- Second aid group shut -

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.

"Mercy Corps believes in the work we have done in northeast Nigeria, and we hope to quickly resume our programs that bring much-needed relief to the people of Nigeria."

An AFP reporter witnessed soldiers camped at the entrance to the Mercy Corps' office in Maiduguri, the capital of Borno State.

The army's crackdown on the international organisations is the latest flashpoint in the tense relations between aid groups and the military.

The army have accused humanitarian organisations of working with jihadists before.

In December 2018 the military suspended UNICEF from operating in the northeast over claims it was training "spies" who were supporting Boko Haram -- only to lift the ban later the same day after a meeting with the aid agency.

"The humanitarian community and the Nigerian authorities have a fraught history," Vincent Foucher, a researcher from France's CNRS, told AFP.

He said the latest closures came amid "a renewed attempt by the army to stifle Boko Haram's war economy.... Could pressure on humanitarian action be part of that larger effort?"

The decade-long insurgency has killed 35,000 people and displaced about two million from their homes in northeast Nigeria.

Government services are often non-existent and the UN says that some 7.1 million people remain in need of humanitarian assistance across the region.

Boko Haram fighters have abducted huge numbers of women and children across the region.

The group drew worldwide attention with the kidnapping of 276 girls from a school in Chibok in 2014.

ICRC workers Hauwa Liman and Saifura Khorsa were murdered by ISWAP last year and a woman working for the UN children's agency UNICEF is also still being held by the group.

The jihadists are also holding 16-year-old Leah Sharibu, the last remaining captive of over 100 schoolgirls kidnapped by jihadists in Dapchi Town, Yobe State, last February. (AFP)

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