Christians have a duty to convert all Muslims, including extremists, but not Jews, according to a Vatican official.

Cardinal Kurt Koch leads ecumenical relations for the Vatican and is one of Pope Francis' senior aides. He spoke at an interfaith meeting at Cambridge University's Woolf Institute.

"We have a mission to convert all non-Christian religions' people [except] Judaism," he told the gathering of Catholic and Jewish leaders.

The cardinal went on to say this mission included jihadis who slaughtered Christians in the Middle East, according to the Catholic Herald.

However, Judaism should be viewed as a "mother" by Christians, the cardinal said, arguing that Christianity and Judaism share a special relationship.

"It is very clear that we can speak about three Abrahamic religions but we cannot deny that the view of Abraham in Jewish and the Christian tradition and the Islamic tradition is not the same," he said.

"In this sense we have only with Jewish people this unique relationship that we do not have with Islam." Jews should be exempt from Christian evangelism because they are a "chosen" people, he said.

The Swiss clergyman was Bishop of Basel before he moved to Rome and serves as the president of the Pontifical Council for Promoting Christian Unity.

His comments follow a document released by the Vatican in December that told Catholics not to try and convert Jews.

The Gifts and Calling of God are Irrevocable marked a significant shift in tone from the Catholic Church which used to refer to Jews as "perfidious" and blind.