Three Egyptian Coptic monks were killed in a "criminal assault" inside a Coptic monastery in South Africa, the Christian Coptic Orthodox Church of Egypt said on Tuesday, and South African police said they were investigating a triple murder.

"Three monks were subjected to a criminal assault inside our Coptic monastery," the spokesman for the Coptic Orthodox Church said in statement posted on Facebook, without elaborating.

A South African police spokesperson said that authorities were investigating the murder of three priests at a church in Cullinan, a town near Pretoria.

All three victims were found with stab wounds while a fourth who survived alleged that he was hit by an iron rod before fleeing and hiding, said police spokesperson Col Dimakatso Nevhuhulwi in a statement.

"The motive (for the) murders is unknown at this stage," he said, adding that the suspects "reportedly left the scene without taking any valuable item(s)".

South Africa has one of the highest murder rates in the world, and armed robbery is common.

One of the three monks killed was a representative of the Coptic Diocese of South Africa, said the church statement.

An internal investigation has been launched, it added, and the Egyptian ambassador to Johannesburg has been informed.