MINYA, Egypt — Islamic militants Friday ambushed three buses carrying Christian pilgrims on their way to a remote desert monastery south of the Egyptian capital of Cairo, killing seven and wounding 19, according to the Coptic Orthodox Church and the Interior Ministry.

All but one of those killed were members of the same family, according to a list of the victims’ names released by the church, which said among the dead were a boy and a girl, age 15 and 12, respectively.

The local Islamic State affiliate which spearheads militants fighting security forces in the Sinai Peninsula claimed responsibility for the attack in a statement, according to SITE, a U.S.-based group that monitors and translates militants’ statements.

It said the attack was revenge for the imprisonment by Egyptian authorities of “our chaste sisters.” It did not elaborate.

The Islamic State has repeatedly vowed to go after Egypt’s Christians as punishment for their support of President Abdel-Fattah el-Sissi.

As defense minister, el-Sissi led the military’s 2013 ouster of an Islamist president, whose one-year rule proved divisive. IS has claimed responsibility for a string of deadly attacks on Christians dating back to December 2016.

The attack is likely to cast a dark shadow on one of el-Sissi’s showpieces – the World Youth Forum – which opens Saturday in the Red Sea resort of Sharm el-Sheikh and hopes to draw thousands of local and foreign youths to discuss upcoming projects, with Egypt’s 63-year-old leader taking center stage.