Pope Francis rested well overnight, the Vatican said on Tuesday, after the 88-year-old pontiff suffered a setback a day earlier as he battles double pneumonia in hospital.

Francis, who has been in Rome's Gemelli Hospital since February 14, had what the Vatican described as two episodes of "acute respiratory insufficiency" on Monday.

They required him to return to using non-invasive, mechanical ventilation to help him breathe.

"The pope slept all night and is continuing to rest," said a one-line note from the Vatican. A full medical update on the pope's condition was expected on Tuesday evening.

Francis has not been seen in public since entering hospital, his longest absence from view since his papacy started in March 2013. His doctors have not said how long his treatment might last.

Monday's setback came following several days of relatively upbeat statements about his condition. The Vatican said the two respiratory episodes on Monday were caused by "a significant accumulation of endobronchial mucus".

The pope, it said, had suffered a bronchospasm, akin to an asthma attack, and had required two bronchoscopies, or procedures to inspect his air passages.

The pope's prognosis remained "guarded", Monday's statement said, which means Francis is not out of danger.

A Vatican official, who did not wish to be named because he was not authorised to discuss the pope's health, said on Monday evening that the pontiff's blood tests that day had remained stable.

The pope's doctors believe the respiratory episode was part of his body's normal response in fighting infection, the official added.

Francis has experienced several bouts of ill health over the last two years and is prone to lung infections because he had pleurisy as a young adult and had part of one lung removed.

Double pneumonia is a serious infection in both lungs that can inflame and scar them, making it difficult to breathe.