As Pope Francis has battled double pneumonia in hospital for nearly three weeks, talk of the 88-year-old pontiff's death or resignation has become almost commonplace.

Pope Francis experienced more respiratory problems and went on noninvasive ventilation on Monday, the Vatican said, as the head of the Roman Catholic Church battles double pneumonia in the hospital.

The 88-year-old pontiff is in good spirits and is grateful for the prayers for his recovery, the Vatican said.