The United Nations Secretary-General, Antonio Guterres, has condemned the killing of five aid workers in Borno State by an armed group suspected to be Boko Haram terrorists.
In a statement signed by his spokesperson, Stephane Dujarric, the UN leader demanded that the perpetrators be brought to book.
The gunmen who carried out the killing said their victims were aid workers working for non-governmental organisations.
The UN had earlier stated that the slain workers were committed humanitarians who devoted their lives to helping vulnerable people and communities in an area heavily affected by violence.
"The Secretary-General strongly condemns the killing, on 22 July, of five civilians, including three aid workers from ACTED, the International Rescue Committee (IRC), and Action Against Hunger (ACF), by an armed group in northern Borno State, northeastern Nigeria," the statement read
Mr Guterres expressed his deepest condolences to the families of the victims and to the Government and people of Nigeria.
He called on the Nigerian authorities to spare no effort in identifying and swiftly bringing to justice the perpetrators of these killings.
"The Secretary-General reiterates that attacks by a party to an armed conflict that are directed against civilians, including those who are aid workers, violate international humanitarian law. International human rights law and international humanitarian law must be fully respected, including the obligation to protect civilians."
President Muhammadu Buhari, the European Union and the United States authorities and a few others have condemned the killing, calling for a proactive action.