Vatican City, Holy See - Pope Francis is spending short moments breathing on his own, the Vatican said on Monday, the day after publishing the first photo of the 88-year-old since his hospitalization a month ago.

The head of the Catholic Church has been in Rome's Gemelli hospital since February 14, treated for pneumonia in both lungs.

For at least a fortnight, Francis has been alternating an oxygen mask at night with a cannula – a plastic tube tucked into his nostrils that delivers high-flow oxygen – during the day.

He is now shifting to a reduced flow for the first time, and "for certain moments, probably short, the pope can go without oxygen," the Vatican press office said in an evening briefing.

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Francis, who has been pope since March 2013, suffered several respiratory crises in the hospital, raising real fears for his life, but has for the past two weeks been improving.

The Vatican press office said Monday that his condition remained stable and that he spent the day in physical therapy, prayer, rest, and some work.

The next formal medical bulletin on his health will be "at the earliest Wednesday evening," it added.

The Vatican on Sunday evening released the first photo of the Argentine since he was admitted, showing him sitting in a wheelchair in the chapel on the 10th floor of the Gemelli, where there is a special papal suite.

It said the picture had been snapped that morning. Taken from behind Francis's right side, his face was not fully visible, but his eyes were open, and he had no cannula.

The pope has not been seen in public for more than a month but has been issuing written statements, including one on Sunday acknowledging his fragile health.

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"I am sharing these thoughts with you while I am facing a period of trial, and I join with so many brothers and sisters who are sick: fragile, at this time, like me," wrote Francis.

"Our bodies are weak but, even like this, nothing can prevent us from loving, praying, giving ourselves, being for each other, in faith, shining signs of hope," he added.

Francis has suffered increasing health issues in recent years, but this is the longest and most serious hospitalization of his papacy.