VATICAN CITY - Doctors have had to change treatment for Pope Francis' respiratory tract infection to tackle a "complex clinical situation" and he will remain in hospital for as long as necessary, the Vatican said on Monday.

"The results of the tests carried out in recent days and today have demonstrated a polymicrobial infection of the respiratory tract, which has led to a further modification of the therapy," said a brief statement.

"All tests conducted up to today are indicative of a complex clinical picture that will require an appropriate hospital stay," it said.

The 88-year-old pontiff has been suffering from a respiratory infection for more than a week and was admitted to Rome's Gemelli hospital on Friday.

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Vatican spokesman Matteo Bruni said the pontiff was "in good spirits". He did not specify whether the pope was suffering from a bacterial or viral infection, but said a further update on the pope's condition would be issued later on Monday.

The pontiff's doctors had earlier ordered complete rest, and Francis was unable to deliver his regular weekly prayer on Sunday to pilgrims in St Peter's Square or lead a special mass for artists to mark the Catholic Church's Jubilee Year.

The Vatican on Sunday described his condition as "stable".

While in hospital over the weekend, the pope continued his recent practice of making phone calls to speak with members of a Catholic parish in Gaza, Italian broadcaster Mediaset reported on Monday.

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One of the parish members said Francis had called on both Friday and Saturday and was in "good spirits" but sounded "a bit tired".

The Vatican has cancelled the pope's visit to Rome's Cinecitta film studios, scheduled for Monday.

"Thank you for the affection, prayer and closeness with which you accompany me in these days," the pope wrote on X on Sunday.