Pope Francis's condition remained stable but complex on Friday as he battles pneumonia in hospital, the Vatican said, a day after releasing an audio message in which the 88-year-old sounded weak and breathless.

The message broadcast to pilgrims in St Peter's Square on Thursday evening was the first time the world had heard the pope's voice since he was admitted to Rome's Gemelli hospital on February 14.

The Argentine pontiff has suffered several respiratory crises since his admission, most recently on Monday.

Amid increasingly lurid online speculation, fuelled by the absence of any photos of the pope, the Holy See on Thursday released a short audio message recorded that day by Francis.

"I thank you from the bottom of my heart for your prayers for my health from the Square. I accompany you from here," the pope said, sounding weak and taking laboured breaths.

"May God bless you and the Virgin protect you. Thank you."

The Argentine spoke in his native Spanish, drawing speculation he could not muster the........

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