The Islamic State West African Province (ISWAP) released a video on Tuesday purportedly showing the execution of eleven hostages, ten Christians plus one Muslim. The group said the murders were in retaliation for the deaths of ISIS leaders Abu Bakr al-Baghdadi and Abu al-Hassan al-Muhajir.

Baghdadi, the supreme leader or “caliph” of the Islamic State, was killed in Syria by U.S. special forces on October 26. The next day, U.S. airstrikes killed ISIS spokesman Abu al-Hassan al-Muhajir.

ISWAP said two additional Muslim hostages were spared from execution. The full group of 13 hostages appeared in a video released on December 17 in which they asked the Christian Association of Nigeria to negotiate with the Islamic State group to secure their release.

The office of U.N. Secretary-General Antonio Guterres issued a statement of concern on Tuesday evening, offering condolences to the families of the victims and expressing the support of the United Nations for Nigeria’s government and people.

“The Secretary-General recalls that attacks by a party to an armed conflict that target civilians, aid workers, and civilian infrastructure violate international humanitarian law. Those responsible for these atrocities must be held accountable,” the statement said.

Within a few hours of the publication of the ISWAP execution video, Islamic State affiliate Boko Haram attacked a predominantly Christian village in Nigeria, killed seven people, and abducted a teenage girl.