Pope Francis spent a peaceful night in hospital, the Vatican said Friday, a day after it noted improvement in his critical clinical condition for a second straight day. 

After being admitted to Rome's Gemelli hospital on 14 February for breathing difficulties, the pontiff sparked alarm across the globe as his condition deteriorated into pneumonia in both lungs. 

"As in the past days, the night has passed quietly and the Pope is now resting," the Vatican said in its morning bulletin on the 88-year-old's health.

After a difficult weekend in which he suffered a respiratory attack, the Vatican said this week that there had been slight improvements in his condition.

It has not, however, modified his prognosis of "reserved," with medical experts saying the pope's age and pre-existing conditions meant a sustained recovery could take longer. 

"Given the complexity of the clinical picture, further days of clinical stability are necessary to resolve the prognosis," the Vatican said Thursday.