Pope Francis holds a mass to mark 500 years of Christianity in the Philippines, on March 14, 2021 at St. Peter's Basilica in The Vatican. Tiziana Fabi, Pool/AFP 

Pope Francis suffered two new breathing attacks on Monday, requiring two separate bronchoscopies, the Vatican said, as the 88-year-old pontiff struggles to recover from pneumonia.

"Today, the Holy Father experienced two episodes of acute respiratory failure, caused by a significant accumulation of endobronchial mucus and consequent bronchospasm," it said in a statement on Francis's 18th day in hospital, the longest of his papacy.

The Argentine 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.

The Vatican said Monday in its nightly medical bulletin that two bronchoscopies were performed on the pope in order to "aspirate abundant secretions".

It said the pope had resumed "non-invasive mechanical ventilation" in the afternoon -- the use of an oxygen mask -- noting that he was "alert, focused and cooperative."

As it has since the start of the pope's hospitalisation, the Vatican said Francis's prognosis remains "reserved," an indication that doctors cannot predict the likely outcome of his condition.

On Sunday evening, the Vatican had said the pope's condition was stable, after he suffered a breathing crisis on Friday.

The leader of the world's almost 1.4 billion Catholics had required the oxygen mask on Friday and Saturday, but not on Sunday, when he participated in mass and spent the rest of the day alternating rest with prayer.

Francis had also on Sunday received Italian Cardinal Pietro Parolin, the Vatican's secretary of state, and Edgar Pena Parra, a Venezuelan archbishop who is also a senior Vatican official.

The Jesuit, who has been pope since March 2013, is being treated in a special suite reserved for pontiffs on the 10th floor of the Gemelli.

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."