Islamic militants have ambushed three buses carrying Christian pilgrims on their way to a remote desert monastery south of the Egyptian capital of Cairo, killing seven people and wounding 19, Egyptian officials said.

All but one of those killed were members of the same family, according to a list of the victims’ names released by the Coptic Orthodox 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 US-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.

It also said the attack left 13 Christians killed and another 18 wounded, but it was not immediately possible to independently verify the claim.

The Islamic State has repeatedly vowed to go after Egypt’s Christians as punishment for their support of president Abdel-Fattah el-Sissi. As defence minister, el-Sissi led the military’s 2013 ouster of an Islamist president, whose one-year rule proved divisive. It has claimed responsibility for a string of deadly attacks on Christians dating back to December 2016.

El-Sissi, who has made the economy and security his top priorities since taking office in 2014, wrote on his Twitter account that Friday’s attack was designed to harm the “nation’s solid fabric” and pledged to continue fighting terrorism.

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 youth to discuss upcoming projects, with Egypt’s 63-year-old leader taking centre stage.

“They want to embarrass el-Sissi and show that the state is unable to protect the Copts,” said Begemy Naseem Nasr, the priest of the church of Saint Mary in Minya. “Egypt is the target here and we all know that.”

Friday’s attack is the second to target pilgrims heading to the St. Samuel the Confessor monastery in as many years, indicating that security measures in place since then are either inadequate or have become lax. The previous attack in May 2017 left nearly 30 people dead.

It is also the latest by IS to target Christians in churches in Cairo, the Mediterranean city of Alexandria and Tanta in the Nile Delta.

Those attacks left at least 100 people dead and led to tighter security around Christian places of worship and Church-linked facilities. They have also underlined the vulnerability of minority Christians in a country where many Muslims have since the 1970s grown religiously conservative.

The Interior Ministry, which oversees the police, said Friday’s attackers used secondary dirt roads to reach the buses carrying the pilgrims, who were near the monastery at the time of the attack. Only pilgrims have been allowed on the main road leading to the monastery since last year’s attack.

The Interior Ministry maintained that only one bus was attacked, but the latest statement by the church said three buses were targeted and put the death toll at 7 and the wounded at 19, including two in critical condition.

The Interior Ministry said police were pursuing the attackers, who fled the scene.

Egypt’s Christians, who account for some 10 percent of the country’s 100 million people, complain of discrimination in the Muslim majority country. Christian activists say the church’s alliance with el-Sissi has offered the ancient community a measure of protection, but failed to end frequent acts of discrimination that boil over into violence against Christians, especially in rural Egypt.

In Minya, the scene of Friday’s attack, Christians constitute the highest percentage of the population – about 35 percent – of any Egyptian province.