Pope Francis, who is battling double pneumonia in hospital, had a "tranquil" night and rested, the Vatican said on Sunday.

The pope was admitted to Rome's Gemelli hospital on February 14 after experiencing difficulty breathing for several days and subsequently had pneumonia diagnosed in both lungs.

The Vatican described his condition as critical for the first time on Saturday, reporting that the 88-year-old Francis had needed supplemental oxygen and blood transfusions that day.

"The night was tranquil, the pope rested," the Vatican said in a one-sentence update on Sunday morning without providing 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", saying it is being caused by two or more microorganisms.

Francis, who has been pope since 2013, has suffered bouts of ill health in 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.

In a statement on Saturday evening, the Vatican said the pope had suffered a "prolonged asthma-like respiratory crisis" that had required administration of "high-flow oxygen".

It said he had also needed blood transfusions because tests showed he had a low platelet count, which is associated with anaemia. Platelets are cell fragments in our blood that form clots and stop or prevent bleeding.

In a briefing on Friday, two of his doctors said the pope was highly vulnerable because of his age and general frailty.

Dr. Sergio Alfieri, a senior member of the Gemelli staff, said there was a risk the lung infection could spread to his bloodstream and develop into sepsis, which "could be very difficult to overcome".