Pope Francis has fuelled the rumour mill with a postponed Africa trip and the curious timing of an upcoming meeting of cardinals -- but experts caution against assuming a resignation is nigh.

Hobbled by pain in his knee and forced to use a wheelchair in recent weeks, the 85-year-old pontiff postponed a July trip to the Democratic Republic of Congo and South Sudan last week.

That move, along with an unusual decision to hold a consistory to name new cardinals during the vacation month of August, triggered intense speculation about his plans for the future, including the most radical -- that he was planning to step down.

Not so fast, many say.

"In the pope's entourage, the majority of people don't really believe in the possibility of a resignation," a Vatican source told AFP.

Rumours within the insular Roman Curia -- the Catholic Church's powerful governing body -- are nothing new, and often fuelled by those with an interest, said Italian Vatican expert Marco Politi.  

"These rumours are encouraged by the pope's opponents who are only eager to see Francis leave," he told AFP.

Door to retired popes

The resignation of a pope was once almost unthinkable. But when Benedict XVI stood down in 2013, citing his declining physical and mental health, he set a precedent.

In 2014, a year after being elected to replace Benedict, Francis himself told reporters that were his health to impede his functions as pope, he would consider stepping down too.

"He (Benedict) opened a door, the door to retired popes," the pontiff said then.

More recently in May, as reported by various Italian media, Francis joked about his knee during a closed-door meeting with bishops: "Rather than operate, I'll resign."

However, a trip to Canada at the end of July is still on the pontiff's schedule, and the pope continues to receive injections in his knee and physical therapy, according to the Vatican.

As a child, Francis had one of his lungs partially removed. Today, besides his knee issue, he suffers recurring sciatic nerve pain.

Rumours of a resignation also flared last year after Francis underwent colon surgery, prompting him to tell a Spanish radio station that the idea "hadn't even crossed my mind".

But beyond his health, a series of calendared events in upcoming months have some Vatican watchers questioning whether Francis is laying the groundwork for retirement, while ensuring that his reforms stay intact.

Convening cardinals

First was his decision to call an extraordinary consistory for August 27, a slow summer month at the Vatican, to create 21 new cardinals -- 16 of whom will be under the age of 80, thereby eligible to elect his successor in a future conclave.

Since becoming pope in 2013, the Argentine pontiff has created 83 cardinals in a move to shape the future of the Catholic Church, in part to counter Europe's historically dominant influence, and to reflect his values.

On August 28, Francis will pay a visit to L'Aquila and the tomb of Celestine V -- the first pope to have resigned from the papacy, in the 13th century.

He then joins the world's cardinals -- many of them meeting their peers for the first time -- in two days of discussions over the reform of the Roman Curia, which Francis announced in March with the unveiling of a new constitution.

Francis' shake-up of the Roman Curia attempts to shift the Church back towards its pastoral roots, allows lay Catholics to head Vatican departments and creates a dicastery specifically for charity works among other reforms.

Could the August calendar be taken as a hint, as some in the media have suggested?

"At this stage, it is a question of being realistic and not alarmist," Politi cautioned.

He said it was "hard to imagine" Francis would resign while the Synod of Bishops -- an initiative meaningful to Francis that is intended to study how the Church moves forward in a more inclusive way -- is ongoing, due to complete in 2023.

Alberto Melloni, a professor of Christianity and secretary of the John XXIII Foundation for Religious Sciences in Bologna, told AFP "preposterous" conjectures had been made about the pope's health and his intentions.

"These are things in which there is a desire to understand, to speculate, but there is little to say," he said.