Pope Francis was doing physiotherapy Saturday after passing a quiet night, the Vatican said, as the 88-year-old head of the global Catholic Church battles pneumonia.

Francis "is going on with treatment and physiotherapy this morning", the Vatican said, having earlier issued a briefing which simply said his night had been "calm".

The pontiff, who had part of one of his lungs removed as a young man, has been in a special papal suite at Rome's Gemelli hospital since February 14.

The Argentine has suffered several respiratory crises since his admission, most recently on Monday, but for several days now the Vatican has described his condition as "stable".

He is still in a "complex clinical condition" so "the prognosis remains guarded", it said Friday.

On Thursday Francis released an audio message -- the first time the world had heard his voice since he went into hospital -- in which he thanked those praying for his recovery.

The pontiff sounded weak and breathless despite marking three weeks of treatment at the Gemelli.

"I thank you from the bottom of my heart for your prayers for my health from the Square. I accompany you from here," said Francis, taking laboured breaths every couple of words.

- Resting, praying -

The pope on Friday mostly rested and prayed, including spending around 20 minutes in the little chapel inside the hospital's papal suite, the Vatican said.

He continues to switch between an oxygen mask at night and a cannula -- a plastic tube tucking into the nostrils -- delivering high-flow oxygen during the day.

He has been doing bits of work when possible.

On Saturday, Italian Cardinal Pietro Parolin read aloud a message on the pope's behalf during a mass in St Peter's Basilica.

The pope had worked on the message from hospital just a few days ago, on March 5, the Vatican said.

In a bid for greater transparency, the Vatican has been publishing an update on how the pope slept every morning, followed by a more detailed medical bulletin each evening.

On Thursday it said that "in view of the stability of the clinical picture", there would be no medical bulletin on Friday evening. The next is due later Saturday.

During previous hospitalizations, the leader of the world's 1.4 billion Catholics appeared on the Gemelli balcony for his weekly Sunday Angelus prayer.

But he has missed the last three.

The Vatican press office said Saturday that it was "quite probable" the Angelus would be delivered this Sunday "in the same way" as in previous weeks, when it was published as a letter released at noon.

The pope has suffered a series of health issues in recent years, from colon surgery in 2021 to a hernia operation in 2023, but this is the longest and most serious hospitalization of his papacy.