Pope Francis's health has deteriorated over the past 24 hours, the Vatican said in a statement on Saturday, adding that he had suffered a "prolonged asthma-like respiratory crisis" and had needed blood transfusions.

"The Holy Father remains alert and has spent the day in a chair, though he is more unwell than yesterday. At the moment, the prognosis remains guarded," the Vatican said.

Francis was admitted to Rome's Gemelli hospital on Feb. 14 after experiencing difficulty breathing for several days. He has since been diagnosed with double pneumonia.

The Vatican said on Saturday that doctors had to administer a "high flow" of oxygen because of his breathing crisis. It added that the blood transfusions were necessary because tests showed he had a low platelet count, associated with anemia.

"The Holy Father's condition remains critical," the statement said. "The Pope is not out of danger."

WATCH | Hope and prayers for Pope's recovery: 

Hope and prayers for Pope's recovery in Rome

Tourists and residents alike expressed their well-wishes for Pope Francis's recovery, in St. Peter's Square at the Vatican and outside Agostino Gemelli University Policlinic in Rome where the pontiff is being treated for pneumonia. On Tuesday night, a group of faithful from Bolivia prayed outside the hospital, below a statue of Pope John Paul II.

The Vatican announced earlier in the day that he would not appear in public on Sunday to lead prayer with pilgrims, the second consecutive week he will have missed the event.

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 micro-organisms.