The masked assailants shot randomly at the people as they left the church, the sources said. It was not immediately clear if those killed were Christians, they said.
State news agency MENA reported that one of the dead was an eight-year-old child.
A Coptic priest at the wedding told Reuters he was inside the church when gunfire broke out. Thomas Daoud Ibrahim said he rushed outside to find a dead man, a dead woman, and “many injured”.
Coptic Christians make up 10 percent of Egypt’s 85 million people, and have generally coexisted peacefully with majority Sunni Muslims for centuries, despite bouts of sectarian tension.
But the army’s overthrow of elected Islamist President Mohamed Mursi on July 3 has been followed by the worst attacks on churches and Christian properties in years.
The immediate trigger for the attacks was a bloody security crackdown in Cairo on August 14, when police dispersed two Islamist protest camps set up to demand the reinstatement of Mursi, and killed hundreds of his supporters.
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(Reporting By Maggie Fick and Omar Fahmy; Writing by Maggie Fick; Editing by Alistair Lyon and Paul Simao)