DAKAR, Senegal — An affiliate of the Islamic State in Nigeria has claimed responsibility for the execution of 11 people, saying the killings were in retaliation for the death of the ISIS leader Abu Bakr al-Baghdadi in Syria in October.

A video released on Thursday showed members of the Nigerian affiliate slashing the throats of 10 people and shooting an additional person. A voice-over says the killings are a “message for Christians” and that all of those killed were Christian, although Nigerian experts said some of them were probably Muslims, based on previous episodes involving the group.

The Islamic State, or ISIS, has lost all of the territory it once held in Iraq and Syria, but it remains a threat even after Mr. al-Baghdadi was killed in an American raid on his hide-out in northwestern Syria. In addition to the affiliate in Nigeria, which is known as the Islamic State West Africa Province, groups in the Philippines, Afghanistan, Sinai and the Sahel, a 3,000-mile stretch of land south of the Sahara, also claim allegiance to ISIS.

The members of the Islamic State West Africa Province, which is known by the acronym ISWAP, left the Islamic militant group Boko Haram in 2016. According to the International Crisis Group, it has between 3,500 and 5,000 fighters. Its leaders split from Boko Haram in part because they disapproved of the violence that the group and its harsh leader, Abubakar Shekau, has meted out to Muslims, according to analysts.