Pope Francis's respiratory tract infection is presenting a "complex clinical picture" that will require further hospital treatment, the Vatican said.
Vatican spokesman Matteo Bruni said the results of tests conducted in recent days and on Monday indicate the pope is suffering from a "polymicrobial respiratory tract infection" that has necessitated a further change in his drug therapy.
There was no timeframe given for his hospital treatment, which has already sidelined him for longer than a 2023 hospital admission for pneumonia.
Concerns are growing about the increasingly frail health of the 88-year-old pontiff.
Mr Bruni said the complexity of his symptoms "will require an appropriate hospital stay".
Francis was admitted to Rome's Gemelli hospital in a "fair" condition on Friday after a week-long bout of bronchitis worsened.
Doctors confirmed a respiratory tract infection and prescribed "absolute rest" alongside unspecified drug therapies.
Subsequent updates said his slight fever had gone away and that he was in a "stable" condition.
Mr Bruni said Francis ate breakfast and read the newspapers on Monday morning after a third peaceful night.
The Argentine pope, who had part of one lung removed after a pulmonary infection as a young man, is a known workaholic who keeps up a gruelling pace despite his increasingly precarious health.
When he had a bad case of pneumonia in 2023, he left after three days and only acknowledged after the fact that he had been admitted urgently after feeling faint and having a sharp pain in his chest.
This time around, Francis insisted on finishing his morning audiences on Friday before leaving the Vatican, even though he was having trouble speaking at length because he was short of breath.
The Holy See has only confirmed cancelled appointments through Monday.
Francis was supposed to have gone to Rome's Cinecitta film studios to meet with artists as part of the Catholic Church's Holy Year celebrations.
While his private audiences are rarely announced with much advance notice, the pope's next scheduled appointment is his weekly general audience on Wednesday.
Beyond that, he is scheduled to preside on Sunday at the ordination of deacons as part of a Holy Year weekend dedicated to deacons.
Francis' participation in both appears in doubt, but they remain on the official Vatican schedule.
When he missed the Jubilee Mass dedicated to artists this past Sunday, a cardinal stood in for him.
