Pope Francis has phoned Coptic Orthodox Pope Tawadros II of Alexandria, expressing his prayers and condolences for a terrorist attack at the Cairo cathedral that left 24 Christians dead.

"We are united in the blood of our martyrs," the Pope told the Orthodox patriarch, according to a Vatican statement.

The patriarch thanked Pope Francis for his closeness at such a sad time and asked his continued prayers for the Copts and for peace in Egypt, the statement said.

On a December weekend bloodied by terrorist attacks in Egypt and Turkey, Pope Francis condemned the violence and urged people to hold fast to their faith and renew their commitment to upholding basic human values.