CAIRO — Egypt faced the prospect of a surge in sectarian bloodshed on Tuesday after the Islamic State claimed responsibility for the bombing of a Coptic cathedral in Cairo that killed 25 people on Sunday. The group warned of more attacks to come.

As Christians gathered in churches to pay their respects to the victims, most of them women, the Islamic State said it had sent a suicide bomber to the chapel on the grounds of St. Mark’s Coptic Orthodox Cathedral, the seat of the Egyptian Orthodox Church.

The Islamic State, also known as ISIS or ISIL, identified the bomber by his nom de guerre, Abu Abdallah al-Masri. The Egyptian authorities on Monday identified the attacker as Mahmoud Shafik Mohamed Mostafa, but said he had used a different alias. There was no immediate explanation for the discrepancy.