Pope Francis, in critical condition battling double pneumonia, had a "good" night in hospital, slept and is resting, the Vatican said on Monday.

The 88-year-old pope was admitted to Rome's Gemelli hospital on February 14 after having trouble breathing for several days, and subsequently had pneumonia diagnosed in both lungs.

The Vatican described his condition as critical for a second day on Sunday and that he had an "initial, slight insufficiency" in his kidney function that day.

The prognosis for the pope, who required a transfusion of two units of blood on Saturday after experiencing a "prolonged asthma-like respiratory crisis," remains "guarded", according to the latest medical update.

"It was a good night, the pope slept and is resting," the Vatican said in a one-sentence update on Monday morning that did not provide further information. 

Double pneumonia is a serious infection that can inflame and scar both lungs, making it difficult to breathe. The Vatican has described the pope's infection as "complex," and that it was caused by two or more microorganisms.

Francis, who has been pope since 2013, has suffered bouts of ill health over the past two years. He is particularly prone to lung infections because he developed pleurisy as a young adult and had part of one lung removed.