Pope Francis had a quiet night in hospital, the Vatican said Sunday, the morning after revealing the 88-year-old was in a "critical" condition.

The Holy See had late Saturday reported the alarming news that the pope, who in hospital since February 14, had suffered a prolonged respiratory attack and required blood transfusions. "The night passed peacefully, the pope rested," the Holy See said in a short update on Sunday morning.

The Argentine pontiff, head of the Catholic Church since 2013, was initially admitted to Rome's Gemelli hospital with bronchitis, but it developed into double pneumonia. "The Holy Father's condition continues to be critical, therefore, as explained (Friday), the pope is not out of danger," the Vatican said in a regular medical bulletin on Saturday evening. "At the moment the prognosis is reserved," it said.

Francis continues to be alert and "spent the day in an armchair even if he was suffering more than" the day before, the Vatican said.