Responding to reports that in the new documentary Francesco, Pope Francis had endorsed same-sex civil unions, the Roman Catholic Archdiocese of Singapore said on Thursday that the Catholic Church's stance on marriage remains unchanged.

Any comment made by Pope Francis that has been recorded in a documentary "is not considered or admissible as an official papal teaching", it said in a statement.

The "Catholic understanding of marriage is defined as the sacrament by which a baptised man and a baptised woman bind themselves for life in lawful marriage", said the Archdiocese, which oversees all Catholic churches in Singapore.

This constant teaching on marriage "remains unchanged regardless of a civil union between two persons of the same sex approved by the state", it added.

"We do not know at the moment what the Holy Father actually said in the interview and the context of what he said," said the Archdiocese, adding that it has not had an official statement or communication from the Vatican.

"What is legal in society is not necessarily moral or licit for Catholics in the teaching of the Church."

Civil unions are legally recognised relationships granted similar legal rights as conventional marriages.

According to reports, Pope Francis had said in the documentary, which premiered at the Rome Film Festival this week: "Homosexual people have a right to be in a family. They are children of God and have a right to a family. Nobody should be thrown out or be made miserable over it. What we have to create is a civil union law. That way they are legally covered. I stood up for that."

The Pope appeared to be referring to when he was archbishop of Buenos Aires and opposed legislation to approve same-sex marriages but supported some kind of legal protection for gay couples' rights.

The remarks have elicited praise from liberals and also calls for urgent clarification from conservatives.

The Catholic Church teaches that homosexual tendencies are not sinful but homosexual acts are. It teaches that homosexuals should be treated with dignity, but is opposed to gay marriage.

After questions arose about the origins of the quote, a Mexican broadcaster on Thursday confirmed that Pope Francis' comments were made during an interview in May last year that was never broadcast in full.

In the documentary, the Pope was also seen to have used the Spanish phrase "convivencia civil", translated in the subtitles as "civil union".

Some Spanish-speaking priests have said the translation is inaccurate and that it is actually better translated as "law of civil convivience" or "civil coexistence". This would mean that Pope Francis was talking about some kind of legal protection but not a homosexual civil union.

But according to Catholic News Agency, Archbishop Victor Manuel Fernandez, a long-time adviser to Pope Francis, clarified that the Pope's phrase is substantially equivalent to the phrase "civil union". He said this view reflects the Pope's stance as a bishop in Argentina that accepting civil unions might be a way to prevent the passage of same-sex marriage laws in the country.