Boko Haram militants have killed five hostages, including four aid workers, who were abducted last month in northeastern Nigeria, a UN spokeswoman told CNN on Thursday.
The aid workers belonged to different humanitarian agencies, and a security personnel member working with the team was among the hostages, said Eve Sabbagh, spokeswoman for the UN's Office for the Coordination of Humanitarian Assistance.
The UN said it had been trying to secure the release of the workers since June -- when they were abducted at a roadblock while traveling between Monguno town and Maiduguri, the capital of Borno state.
Edward Kallon, UN agency coordinator in Nigeria, said that aid workers were increasingly becoming the target of armed groups in the region's violent attacks.
"I strongly condemn all violence targeting aid workers and the civilians they are assisting. I am also troubled by the number of illegal vehicular checkpoints set up by non-state armed groups along main supply routes," Kallon said in the statement.
The International Rescue Committee said it was deeply saddened by the news, and that one of its staff was among those killed.
"We condemn this barbaric act," the agency said, calling on the killers to return the worker's remains to his family.
President Muhammadu Buhari has also sent his sympathies to the workers' families and the agencies, his media aide Garba Shehu said in a statement.
Buhari said the workers were killed by Boko Haram members and promised that his administration was working to "wipe out" the militants from the country's northeastern region.
"He assures them that security agencies in the state will work closely with their organizations to implement measures to ensure that no such kidnapping of staff occurs again," Shehu said.
Boko Haram militants and jihadist groups operating in northeastern Nigeria have killed and abducted aid workers during more than a decade of violence.
According to international NGO Action Against Hunger, an armed group claimed they had executed five humanitarian workers held captive by militants for many months in 2019.
In 2018, two nurses with the International Committee of the Red Cross were executed by Boko militants after failed negotiations with the Nigerian government for their release.