Retired Pope Benedict XVI has issued a defence of priestly celibacy in the Catholic Church as his successor considers easing a ban on married men serving as priests.

Pope Benedict made the appeal in a book co-authored with Cardinal Robert Sarah.

It comes in response to a proposal to allow married men to be ordained as priests in the Amazon region.

Pope Benedict, who retired in 2013, said he could not remain silent on the issue.

In the book, Pope Benedict says celibacy, a centuries-old tradition within the Church, has "great significance" because it allows priests to focus on their duties.

The 92-year-old says "it doesn't seem possible to realise both vocations [priesthood and marriage] simultaneously".

It is rare for Pope Benedict, who was the first pontiff to resign in almost 600 years, to intervene in clerical matters.

The Vatican is yet to comment on the book, which was previewed in part by French newspaper Le Figaro before its full publication on Monday.

Vatican commentators have reacted with surprise to Benedict's intervention, suggesting it breaks with convention.

"Benedict XVI is really not breaking his silence because he (and his entourage) never felt bound to that promise. But this is a serious breach," Massimo Faggioli, a historian and theologian at Villanova University, tweeted.

The comments by Pope Benedict were described as "incredible" by Joshua McElwee, a journalist for the National Catholic Reporter.

A theological conservative with traditional views on Catholic values, Pope Benedict pledged to remain "hidden from the world" when he retired, citing poor health.

But since then, he has made his views known in articles, books and interviews, advocating a different approach to Pope Francis, who is seen as more progressive. Pope Benedict still lives within the walls of the Vatican in a former monastery.

What is the proposed change to priestly celibacy?

In October, Catholic bishops from around the world gathered for a meeting, known as the synod, to discuss the future of the Church in the Amazon.

At the conclusion of that meeting, a document detailing issues affecting the Church was released. In it, there was a suggestion that in remote parts of the Amazon, older, married men should be ordained.

South American bishops have advocated this in an effort to address the shortage of priests in the region.

Pope Francis will consider it, along with many other proposals, including the environment and the role of women in the Church. He is expected to make a decision on the matter within the next few months.

Why is it controversial?

For many, celibacy is a key part of being a Catholic priest. A priest is supposed to be married to God and not be distracted by what some consider to be worldly concerns like a wife or a family.

For traditionalists, this is about the direction in which Pope Francis is taking the Church.

Some critics regard the idea of allowing married priests in the Amazon as a pretext to abolishing celibacy as a requirement altogether.