(AFP) - Pope Francis opens a major congress on the future of the Catholic Church Wednesday marked by tensions with conservatives on issues such as the treatment of divorcees and LGBTQ believers.

The assembly of the Synod of Bishops will take place over four weeks in Rome, in the wake of an unprecedented two-year global consultation that will also address topics such as women deacons and priestly celibacy.

But even before proceedings began, five conservative cardinals publicly asked Francis to reaffirm Catholic doctrine on the treatment of gay couples and the ordination of women.

Their questions -- entitled "Doubts" -- were accompanied by an open letter to followers warning of the risk of "confusion" and "error", amid criticism that the questions raised through the Synod process could alienate many Catholics.

In a response made public Monday, the 86-year-old pontiff appeared to suggest a way for the blessing of same-sex couples by clerics, something not recognised by the Holy See but practised in countries including Germany and Belgium.

While insisting that the Church only recognises marriage between a man and a woman, the pope said that "we cannot be judges who only deny, reject, and exclude".

"Pastoral prudence must adequately discern whether there are forms of blessing, requested by one or more persons, that do not convey a mistaken concept of marriage," he wrote.

- Chatter and polarisation -

The 16th Ordinary General Assembly opens at 09:00 am (0700 GMT) on Wednesday with a mass in St Peter's Square at the Vatican overseen by the pope, who will then speak later in the afternoon before discussions formally begin.

Since taking office in 2013, Francis has worked to reform the governance of the Church, which he wants to become less top-down and closer to the faithful -- although the way he has sought to do this has met strong internal resistance.

For the first time in the history of the Church, nuns and laywomen will take part in the General Assembly consultations and be able to vote.

One informed observer of the Holy See, who asked not to be named, said their participation will make the synodal process more effective.

"Within the bishops, there is an ecclesiastical culture. With the laity, that won't work anymore, they won't be satisfied with nice words, there will be a demand for procedure, the will to change, efficiency," the source told AFP.

"In this sense, Francis is pushing the boundaries, which is why many are afraid."

A second session of the assembly is scheduled for October 2024, meaning that no concrete decisions are expected any time soon.

But expectations are high, as are concerns, with the consultations so far having highlighted differing views between national Churches, and between them and the Vatican.

Pope Francis warned on Saturday of the need for Catholics to "walk together", calling on the Synod to be above "chatter, ideology and polarisation".

© Agence France-Presse