LAGOS: Jihadists have killed five aid workers abducted last month in restive northeast Nigeria, humanitarian group Action Against Hunger said Wednesday.

The announcement came after insurgents affiliated to the so-called Islamic State group released a video online purporting to show the execution of the five Nigerian men.

"It is with profound grief that we confirm the murder of Ishaku Yakubu, Action Against Hunger (ACF) staff member from Monguno, and four other aid workers," the French-based aid group said.

It said the humanitarian workers "were taken hostage by a non-state armed group in Borno State, northeast Nigeria, on June 8".

"This unjustifiable killing reflects the immense difficulty faced by independent and impartial humanitarian actors, and the violence we are exposed to every day to fulfil our mission," ACF said.

Aid group International Rescue Committee (IRC) confirmed that one of its employees was also among those killed.

"The IRC strongly condemns the senseless execution of our colleague, Luka Filibus, and his fellow humanitarian captives," it said.

"We condemn this barbaric treatment and demand the immediate return of his remains to his family."

Humanitarian workers have been repeatedly targeted by jihadist groups waging a decade-long insurgency in northeast Nigeria.

The 10-year conflict has killed 36,000 people and displaced about two million from their homes in northeast Nigeria.

Last year fighters from the Islamic State West Africa Province (ISWAP) faction abducted a group of six humanitarian workers -- including a female ACF employee -- in the region.

Five of the hostages were later executed and the ACF worker remains in captivity.

ISWAP splintered from jihadist group Boko Haram in 2016 and swore allegiance to then IS leader Abu Bakr al-Baghdadi.

Aid groups provide a vital lifeline for some 7.9 million people who the United Nations says are in need of urgent assistance in the region.