Coptic Christians in the Egyptian town of Minya south of the capital of Cairo are preparing to bury their dead.
A similar attack took place in May 2017, when gunmen fired at a bus carrying Christians to the same monastery, killing at least 28 people. It was carrying a family that had traveled to the monastery for the baptism of one of its children, according to the spokesman.
The Islamic State has repeatedly vowed to go after Egypt's Christians as punishment for their support of President Abdel-Fattah el-Sissi.
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 created to harm the "nation's solid fabric" and pledged to continue fighting terrorism.
"While we extend our heartfelt condolences to the government and people of Egypt on the victims of this awful terrorist attack, we confirm our confidence in the measures taken by the Egyptian government to fight terrorism and whoever espouses it".
Almost 30 people died in the attack on May 26, 2017, in which militants dressed as security officers ordered the bus to stop. The source said authorities were still searching for the attackers.
The following April, 45 people were killed as Christians gathered to celebrate Palm Sunday in the cities of Tanta and Alexandria.
Egypt is a Muslim-majority country and its Christian minority - mostly members of the Coptic Orthodox Church - make up around 10% of the population.
The Interior Ministry said police were pursuing the attackers, who fled the scene.
Copts have long complained of discrimination in Egypt and IS is not the only group to have launched sectarian attacks against the community.
There was no immediate claim of responsibility for the attack.
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