Pope Francis hit the three-week mark Friday in his hospitalization for double pneumonia, in stable condition, receiving therapy but not necessarily improving, after giving the world a tangible indication of just how frail and sick he is.

The 88-year-old Francis spent 20 minutes in the Gemelli hospital chapel, praying and doing some work in between rest and respiratory and physical therapy, the Vatican said. He was using high flows of supplemental oxygen to help him breathe during the day and a noninvasive mechanical ventilation mask at night.

Doctors not involved in his care said after three weeks of acute care in the hospital for double pneumonia, they would have hoped to have seen improvement and warned that he was increasingly at risk of secondary infections the longer he remains hospitalized.

Additionally, Francis has had episodes of acute respiratory failure earlier this week and underwent bronchoscopies to suction mucus from his lungs.

“He’s had respiratory failure and they were not able to liberate him from the hospital in the first three weeks. And therefore I think you’d say this does look concerning, perhaps more concerning than it did right at the beginning,” said Dr. Andrew Chadwick, a respiratory and intensive care specialist at Oxford University Hospitals in England.

Click to play video: 'Pope Francis Making Slow Recovery from Double Pneumonia'

Dr. Jeffrey Millstein, a clinical assistant professor of internal medicine at the University of Pennsylvania, said it wasn’t shocking that Francis hadn’t improved in three weeks, and that it was encouraging he was able to breathe part of the day with just a nasal tube of high-flow oxygen. But he said that his condition certainly was “a precarious, touch and go kind of situation” and that recovery, while still possible, would be a long process.

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Going forward, “I just would be looking for no new setbacks,” he said. “I think as long as he is dealing with the current issues and he’s just making incremental progress, that would be great.”

Francis offered a first public sign of just how weak he is on Thursday by recording an audio message that was broadcast to the faithful in St. Peter’s Square who had gathered for the nightly recitation of the rosary prayer.

In it, Francis thanked the people for their prayers. But his voice was barely discernible through his labored breaths and he spoke in his native Spanish, not Italian.

“I thank you from the bottom of my heart for your prayers for my health from the square, I accompany you from here,” he said to the hushed square. “May God bless you and the Virgin protect you. Thank you.”

Click to play video: 'Pope calls meeting with cardinals as health remains critical'

The messaged served many purposes. It was the first public sign of life from the pope since he entered Rome’s Gemelli hospital on Feb. 14, and put to rest right-wing conspiracy theories and rumors calling into question his true medical status.

The Vatican said that Francis himself wanted to record it, to thank all those people who had been praying for him. In his Sunday message last weekend, Francis said he had felt the affection of so many people, and felt “as if I am ‘carried’ and supported by all God’s people.”

But the audio also underscored just how weak Francis is. For anyone used to hearing his voice, the message was an emotional punch to the gut that hammered home just how hard it is for him to even breathe.

The pope has chronic lung disease and had part of one lung removed as a young man.

The Vatican has given twice-daily updates on Francis’ condition, but has distributed no photos or video of him since the morning of Feb. 14, when he held a handful of audiences at the Vatican before being admitted to Gemelli hospital for what was then just a bad case of bronchitis.

The infection progressed into a complex respiratory tract infection and double pneumonia that has sidelined Francis for the longest period of his 12-year papacy and raised questions about the future of his papacy.