Vatican City - Pope Francis recorded and released an audio message on Thursday thanking those around the world who have been praying for his recovery, his voice breathless as he nears three weeks in hospital with pneumonia.

"I thank you from the bottom of my heart for your prayers for my health from the Square, I accompany you from here," Francis said in a message broadcast in St Peter's Square.

"May God bless you and the Virgin protect you. Thank you," he said, taking labored breaths as he spoke in his native Spanish, with some words fading away into nothing.

It was the first time the world has heard Francis's voice since the 88-year-old was admitted to Rome's Gemelli hospital on February 14.

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Pilgrims have been gathering in St Peter's Square every evening to pray for the pope's recovery. The hundreds of people there on Thursday applauded when they heard his message.

The Vatican said earlier Thursday that the Argentine, head of the worldwide Catholic Church since 2013, is in a "stable" condition.

There had been no repeat of Monday's respiratory failure, it said, and the pope's blood work "remained stable".

Francis continued with his breathing exercises and physiotherapy, did not have a fever, and managed to do a bit of work in both the morning and afternoon, it said.

The Vatican has been providing twice daily updates on the pope's health, a morning one on how the night went, and an evening medical bulletin.

But on Thursday it said that "in view of the stability of the clinical picture, the next medical bulletin will be released on Saturday".

Nonetheless, "the doctors are still maintaining a reserved prognosis", it said, meaning they will not say how they expect his condition to evolve.