Pope 'slept all night' after two breathing attacks: Vatican

The 88-year-old pope was admitted to Rome's Gemelli hospital on February 14 with bronchitis, which developed into pneumonia in both lungs, sparking alarm across the globe.

On Monday, the pope "experienced two episodes of acute respiratory failure, caused by a significant accumulation of endobronchial mucus and consequent bronchospasm," the Vatican had said in its evening update.

Acute respiratory failure, which can be life-threatening, occurs when the lungs cannot pass enough oxygen into the blood, or when carbon dioxide builds up in the body.

It added that Francis's prognosis remains "reserved," an indication that doctors cannot predict the likely outcome of his condition.

The Jesuit is being treated in a special suite reserved for pontiffs on the 10th floor of the Gemelli, where the Vatican has said he has continued to work.

Francis, born Jorge Bergoglio, missed his traditional Angelus prayer for a third straight Sunday and the Vatican issued a written text instead.

In it, the pope thanked well-wishers for their prayers, saying: "I feel all your affection and closeness and, at this particular time, I feel as if I am 'carried' and supported by all God's people. Thank you all."