VATICAN CITY - 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.

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

"The night was tranquil, the pope rested," the Vatican said in its latest update on Sunday morning.

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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.