Pope Francis, who has been hospitalised for two weeks battling double pneumonia, has spent another peaceful night and is now resting, the Vatican said on Friday.

Francis was admitted to Rome's Gemelli hospital on February 14 with what was originally described as a bronchitis and was later revealed to have developed into something more serious.

In a medical update on Thursday, the Vatican said the pontiff's condition had "continued to show improvement" but his prognosis remained guarded due to a complex clinical picture.

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

Double pneumonia is a serious infection of both lungs that can inflame and scar them, making it difficult to breathe. The Vatican said Francis suffered a "prolonged asthma-like respiratory crisis" on Saturday, but there have been no repeats.