Pope Francis, hospitalised for bronchitis four days ago, faces a "complex clinical picture", the Vatican said on Monday, indicating that the 88-year-old's stay will be longer than initially believed.

After initially cancelling events through till Monday, the Vatican said the pope's weekly Wednesday audience would not go ahead either.

"The results of the tests carried out in recent days and today have demonstrated a polymicrobial infection of the respiratory tract that has led to a further change in treatment," the Vatican said in a statement.

"All the tests carried out until now are indicative of a complex clinical picture that will require adequate hospitalisation."

A Vatican source told AFP the pope was receiving oxygen.

A separate source in the pope's entourage had earlier insisted there was "no alarmism" following his Friday admission to Rome's Gemelli hospital.

Vatican spokesman Matteo Bruni told reporters as the new health update was published Monday that he was in "good humour".

But the developments will raise fresh concerns about the Argentine pontiff, who has suffered increasing health issues in recent years.

'Weakened'

Before his admission, after his usual Friday morning meetings, Francis had admitted to difficulty breathing and had asked aides several times to read his speeches aloud on his behalf.

At his weekly general audience last Wednesday, he said he "cannot yet" read his own speeches, adding with a smile: "I hope that next time I can."

A source within the pope's entourage told AFP on Monday that Francis was admitted after a "very busy" two weeks, during which "he was weakened".

But the source added: "There is no alarmism."

Earlier, Bruni said Francis had passed a third "peaceful" night in hospital and on Monday morning had eaten breakfast and read the newspapers.

The pontiff, who had part of one of his lungs removed as a young man, has been plagued by health issues but continues to maintain a very busy schedule.

He pulled out of a Good Friday event last year after catching what the Vatican called a "light flu" but went on to lead Easter services as planned.

A year earlier, in March 2023, Francis was admitted to hospital for three nights with bronchitis, which was cured with antibiotics.

Another bout of bronchitis caused him to cancel a December 2023 visit to Dubai to participate in the United Nations COP28 climate change conference.

The pope also underwent a hernia operation in June 2023 and in 2021 underwent surgery for a type of diverticulitis, an inflammation of pockets that develop in the lining of the intestine.

He has been using a wheelchair since 2022 because of persistent knee pain and uses a cane during rare moments standing up.

Francis has also fallen a couple of times in the past few months, bruising his forearm in January and sporting a large bruise on his right jaw in December, caused by toppling from his bed.

Yet despite his health troubles, Francis remains an active pontiff. 

In September 2024, he completed a four-nation Asia-Pacific tour, the longest of his papacy by duration and distance.