Boko Haram militants have killed five hostages, including four aid workers, who were abducted last month in northeastern Nigeria, a UN spokeswoman stated on Thursday.

Axar.az reports citing foreign media that 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 travelling between Monguno town and Maiduguri, the capital of Borno state.

Edward Kallon, the UN agency coordinator in Nigeria, said that aid workers were increasingly becoming the target of armed groups in the region's violent attacks.